* Filter certain DOM attributes (e.g. src, href) if their values are empty strings
This prevents e.g. <img src=""> from making an unnecessar HTTP request for certain browsers.
* Expanded warning recommendation
* Improved error message
* Further refined error message
* Add feature flag
* Split stack from current fiber
You can get stack from any fiber, not just current.
* Refactor description of component frames
These should use fiber tags for switching. This also puts the relevant code
behind DEV flags.
* We no longer expose StrictMode in component stacks
They're not super useful and will go away later anyway.
* Update tests
Context is no longer part of SSR stacks. This was already the case on the
client.
forwardRef no longer is wrapped on the stack. It's still in getComponentName
but it's probably just noise in stacks. Eventually we'll remove the wrapper
so it'll go away anyway. If we use native stack frames they won't have this
extra wrapper.
It also doesn't pick up displayName from the outer wrapper. We could maybe
transfer it but this will also be fixed by removing the wrapper.
* Forward displayName onto the inner function for forwardRef and memo in DEV
This allows them to show up in stack traces.
I'm not doing this for lazy because lazy is supposed to be called on the
consuming side so you shouldn't assign it a name on that end. Especially
not one that mutates the inner.
* Use multiple instances of the fake component
We mutate the inner component for its name so we need multiple copies.
* ReactFiberReconciler -> ReactFiberReconciler.old
* Set up infra for react-reconciler fork
We're planning to land some significant refactors of the reconciler.
We want to be able to gradually roll out the new implementation side-by-
side with the existing one. So we'll create a short lived fork of the
react-reconciler package. Once the new implementation has stabilized,
we'll delete the old implementation and promote the new one.
This means, for as long as the fork exists, we'll need to maintain two
separate implementations. This sounds painful, but since the forks will
still be largely the same, most changes will not require two separate
implementations. In practice, you'll implement the change in the old
fork and then copy paste it to the new one.
This commit only sets up the build and testing infrastructure. It does
not actually fork any modules. I'll do that in subsequent PRs.
The forked version of the reconciler will be used to build a special
version of React DOM. I've called this build ReactDOMForked. It's only
built for www; there's no open source version.
The new reconciler is disabled by default. It's enabled in the
`yarn test-www-variant` command. The reconciler fork isn't really
related to the "variant" feature of the www builds, but I'm piggy
backing on that concept to avoid having to add yet another
testing dimension.
* Implemented Profiler onCommit() and onPostCommit() hooks
* Added enableProfilerCommitHooks feature flag for commit hooks
* Moved onCommit and onPassiveCommit behind separate feature flag
* Add options for forked entry points
We currently fork .fb.js entry points. This adds a few more options.
.modern.fb.js - experimental FB builds
.classic.fb.js - stable FB builds
.fb.js - if no other FB build, use this for FB builds
.experimental.js - experimental builds
.stable.js - stable builds
.js - used if no other override exists
This will be used to have different ES exports for different builds.
* Switch React to named exports
* Export named exports from the export point itself
We need to re-export the Flow exported types so we can use them in our code.
We don't want to use the Flow types from upstream since it doesn't have the non-public APIs that we have.
This should be able to use export * but I don't know why it doesn't work.
This actually enables Flow typing of React which was just "any" before.
This exposed some Flow errors that needs fixing.
* Create forks for the react entrypoint
None of our builds expose all exports and they all differ in at least one
way, so we need four forks.
* Set esModule flag to false
We don't want to emit the esModule compatibility flag on our CommonJS
output. For now we treat our named exports as if they're CommonJS.
This is a potentially breaking change for scheduler (but all those apis
are unstable), react-is and use-subscription. However, it seems unlikely
that anyone would rely on this since these only have named exports.
* Remove unused Feature Flags
* Let jest observe the stable fork for stable tests
This lets it do the negative test by ensuring that the right tests fail.
However, this in turn will make other tests that are not behind
__EXPERIMENTAL__ fail. So I need to do that next.
* Put all tests that depend on exports behind __EXPERIMENTAL__
Since there's no way to override the exports using feature flags
in .intern.js anymore we can't use these APIs in stable.
The tradeoff here is that we can either enable the negative tests on
"stable" that means experimental are expected to fail, or we can disable
tests on stable. This is unfortunate since some of these APIs now run on
a "stable" config at FB instead of the experimental.
* Switch ReactDOM to named exports
Same strategy as React.
I moved the ReactDOMFB runtime injection to classic.fb.js
Since we only fork the entrypoint, the `/testing` entrypoint needs to
be forked too to re-export the same things plus `act`. This is a bit
unfortunate. If it becomes a pattern we can consider forking in the
module resolution deeply.
fix flow
* Fix ReactDOM Flow Types
Now that ReactDOM is Flow type checked we need to fix up its types.
* Configure jest to use stable entry for ReactDOM in non-experimental
* Remove additional FeatureFlags that are no longer needed
These are only flagging the exports and no implementation details so we
can control them fully through the export overrides.
Adds a feature flag for when React.jsx warns you about spreading a key into jsx. It's false for all builds, except as a dynamic flag for fb/www.
I also included the component name in the warning.
* Split recent passive effects changes into 2 flags
Separate flags can now be used to opt passive effects into:
1) Deferring destroy functions on unmount to subsequent passive effects flush
2) Running all destroy functions (for all fibers) before create functions
This allows us to test the less risky feature (2) separately from the more risky one.
* deferPassiveEffectCleanupDuringUnmount is ignored unless runAllPassiveEffectDestroysBeforeCreates is true
* Build both stable and experimental WWW builds
* Flip already experimental WWW flags to true
* Remove FB-specific internals from modern FB builds
We think we're not going to need these.
* Disable classic features in modern WWW builds
* Disable legacy ReactDOM API for modern WWW build
* Don’t include user timing in prod
* Fix bad copy paste and add missing flags to test renderer
* Add testing WWW feature flag file
We need it because WWW has a different meaning of experimental now.
This PR introduces adds `react/testing` and `react-dom/testing`.
- changes infra to generate these builds
- exports act on ReactDOM in these testing builds
- uses the new test builds in fixtures/dom
In the next PR -
- I'll use the new builds for all our own tests
- I'll replace usages of TestUtils.act with ReactDOM.act.
* Flush useEffect clean up functions in the passive effects phase
This is a change in behavior that may cause broken product code, so it has been added behind a killswitch (deferPassiveEffectCleanupDuringUnmount)
* Avoid scheduling unnecessary callbacks for cleanup effects
Updated enqueuePendingPassiveEffectDestroyFn() to check rootDoesHavePassiveEffects before scheduling a new callback. This way we'll only schedule (at most) one.
* Updated newly added test for added clarity.
* Cleaned up hooks effect tags
We previously used separate Mount* and Unmount* tags to track hooks work for each phase (snapshot, mutation, layout, and passive). This was somewhat complicated to trace through and there were man tag types we never even used (e.g. UnmountLayout, MountMutation, UnmountSnapshot). In addition to this, it left passive and layout hooks looking the same after renders without changed dependencies, which meant we were unable to reliably defer passive effect destroy functions until after the commit phase.
This commit reduces the effect tag types to only include Layout and Passive and differentiates between work and no-work with an HasEffect flag.
* Disabled deferred passive effects flushing in OSS builds for now
* Split up unmount and mount effects list traversal
* Add feature flags
* Add Chunk type and constructor
* Wire up Chunk support in the reconciler
* Update reconciler to reconcile Chunks against the render method
This allows the query and args to be updated.
* Drop the ref. Chunks cannot have refs anyway.
* Add Chunk checks in more missing cases
* Rename secondArg
* Add test and fix lazy chunks
Not really a supported use case but for consistency I guess.
* Fix fragment test
There are two similar flags, `debugRenderPhaseSideEffects` and
`debugRenderPhaseSideEffectsForStrictMode`. The strict mode one is the
only one that is actually used. I think originally the theory is that
we would one day turn it on for all components, even outside strict
mode. But what we'll do instead is migrate everyone to strict mode.
The only place `debugRenderPhaseSideEffects` was being used was in
an internal test file. I rewrote those tests to use public APIs.
* Tests run in experimental mode by default
For local development, you usually want experiments enabled. Unless
the release channel is set with an environment variable, tests will
run with __EXPERIMENTAL__ set to `true`.
* Remove concurrent APIs from stable builds
Those who want to try concurrent mode should use the experimental
builds instead.
I've left the `unstable_` prefixed APIs in the Facebook build so we
can continue experimenting with them internally without blessing them
for widespread use.
* Turn on SSR flags in experimental build
* Remove prefixed concurrent APIs from www build
Instead we'll use the experimental builds when syncing to www.
* Remove "canary" from internal React version string
* Don't bother including `unstable_` in error
The method names don't get stripped out of the production bundles
because they are passed as arguments to the error decoder.
Let's just always use the unprefixed APIs in the messages.
* Set up experimental builds
The experimental builds are packaged exactly like builds in the stable
release channel: same file structure, entry points, and npm package
names. The goal is to match what will eventually be released in stable
as closely as possible, but with additional features turned on.
Versioning and Releasing
------------------------
The experimental builds will be published to the same registry and
package names as the stable ones. However, they will be versioned using
a separate scheme. Instead of semver versions, experimental releases
will receive arbitrary version strings based on their content hashes.
The motivation is to thwart attempts to use a version range to match
against future experimental releases. The only way to install or depend
on an experimental release is to refer to the specific version number.
Building
--------
I did not use the existing feature flag infra to configure the
experimental builds. The reason is because feature flags are designed
to configure a single package. They're not designed to generate multiple
forks of the same package; for each set of feature flags, you must
create a separate package configuration.
Instead, I've added a new build dimension called the **release
channel**. By default, builds use the **stable** channel. There's
also an **experimental** release channel. We have the option to add more
in the future.
There are now two dimensions per artifact: build type (production,
development, or profiling), and release channel (stable or
experimental). These are separate dimensions because they are
combinatorial: there are stable and experimental production builds,
stable and experimental developmenet builds, and so on.
You can add something to an experimental build by gating on
`__EXPERIMENTAL__`, similar to how we use `__DEV__`. Anything inside
these branches will be excluded from the stable builds.
This gives us a low effort way to add experimental behavior in any
package without setting up feature flags or configuring a new package.
* Add trusted types to react on client side
* Implement changes according to review
* Remove support for trusted URLs, change TrustedTypes to trustedTypes
* Add support for deprecated trusted URLs
* Apply PR suggesstions
* Warn only once, remove forgotten check, put it behind a flag
* Move comment
* Fix PR comments
* Fix html toString concatenation
* Fix forgotten else branch
* Fix PR comments
When React schedules a rendering task, it passes a `timeout` option
based on its expiration time. This is intended to avoid starvation
by other React updates. However, it also affects the relative priority
of React tasks and other Scheduler tasks at the same level, like
data processing.
This adds a feature flag to disable passing a `timeout` option to
Scheduler. React tasks will always append themselves to the end of
the queue, without jumping ahead of already scheduled tasks.
This does not affect the order in which React updates within a single
root are processed, but it could affect updates across multiple roots.
This also doesn't remove the expiration from Scheduler. It only means
that React tasks are not given special treatment.
* Add a feature flag to disable legacy context
* Address review
- invariant -> warning
- Make this.context and context argument actually undefined
* Increase test coverage for lifecycles
* Also disable it on the server is flag is on
* Make this.context {} when disabled, but function context is undefined
* Move checks inside
In this PR, for tests (specifically, code inside an `act()` scope), we immediately trigger work that would have otherwise required a timeout. This makes it simpler to tests loading/spinner states, and makes tests resilient to changes in React.
For some of our tests(specifically, ReactSuspenseWithNoopRenderer-test.internal), we _don't_ want fallbacks to immediately trigger, because we're testing intermediate states and such. Added a feature flag `flushSuspenseFallbacksInTests` to disable this behaviour on a per case basis.
Concurrent/Batched mode tests should always be run with a mocked scheduler (v17 or not). This PR adds a warning for the same. I'll put up a separate PR to the docs with a page detailing how to mock the scheduler.
This adds a 'SuspenseCallback' feature flag. When the property is set on
a suspense component it will be called during the commit phase with a
set of the immediate thenable for this component. This will allow user
code to build runtime tracing of the cause for a suspense boundary.
* [Events] Add EventPriority enum
React DOM's DispatchConfig for synthetic events has an `isDiscrete`
field that affects how updates triggered by an event are scheduled.
Events are either discrete or continuous.
This commit adds an additional type of configuration where an event
has user-blocking priority, but is not discrete. E.g. updates triggered
by hover are more important than the default, but they don't need to
be processed serially. Because there are now three types of event
priority instead of two, I've replaced the `isDiscrete` boolean with an
enum: `eventPriority`.
This commit implements the new enum value but does not change any
behavior. I'll enable it behind a feature flag in the next commit.
I've only implemented this in the legacy event system. I'll leave Flare
for a follow-up.
* enableUserBlockingEvents feature flag
Adds a feature flag to increase the priority of events like `mouseover`,
without making them discrete.
* s/flushPassiveEffects/unstable_flushWithoutYielding
a first crack at flushing the scheduler manually from inside act(). uses unstable_flushWithoutYielding(). The tests that changed, mostly replaced toFlushAndYield(...) with toHaveYielded(). For some tests that tested the state of the tree before flushing effects (but still after updates), I replaced act() with bacthedUpdates().
* ugh lint
* pass build, flushPassiveEffects returns nothing now
* pass test-fire
* flush all work (not just effects), add a compatibility mode
of note, unstable_flushWithoutYielding now returns a boolean much like flushPassiveEffects
* umd build for scheduler/unstable_mock, pass the fixture with it
* add a comment to Shcduler.umd.js for why we're exporting unstable_flushWithoutYielding
* run testsutilsact tests in both sync/concurrent modes
* augh lint
* use a feature flag for the missing mock scheduler warning
I also tried writing a test for it, but couldn't get the scheduler to unmock. included the failing test.
* Update ReactTestUtilsAct-test.js
- pass the mock scheduler warning test,
- rewrite some tests to use Scheduler.yieldValue
- structure concurrent/legacy suites neatly
* pass failing tests in batchedmode-test
* fix pretty/lint/import errors
* pass test-build
* nit: pull .create(null) out of the act() call
PR #15650 is a bugfix but it's technically a semantic change that could
cause regressions. I don't think it will be an issue, since the
previous behavior was both broken and incoherent, but out of an
abundance of caution, let's wrap it in a flag so we can easily revert
it if necessary.
Adds a feature flag `enableNewScheduler` that toggles between two
implementations of ReactFiberScheduler. This will let us land changes in
master while preserving the ability to quickly rollback.
Ideally this will be a short-lived fork. Once we've tested the new
scheduler for a week or so without issues, we will get rid of it. Until
then, we'll need to maintain two parallel implementations and run tests
against both of them. We rarely land changes to ReactFiberScheduler, so
I don't expect this will be a huge burden.
This commit does not implement anything new. The flag is still off and
tests run against the existing implementation.
Use `yarn test-new-scheduler` to run tests against the new one.
* Prevent javascript protocol URLs
* Just warn when disableJavaScriptURLs is false
This avoids a breaking change.
* Allow framesets
* Allow <html> to be used in integration tests
Full document renders requires server rendering so the client path
just uses the hydration path in this case to simplify writing these tests.
* Detect leading and intermediate characters and test mixed case
These are considered valid javascript urls by browser so they must be
included in the filter.
This is an exact match according to the spec but maybe we should include
a super set to be safer?
* Test updates to ensure we have coverage there too
* Fix toString invocation and Flow types
Right now we invoke toString twice when we hydrate (three times
with the flag off). Ideally we should only do it once even in this case
but the code structure doesn't really allow for that right now.
* s/itRejects/itRejectsRendering
* Dedupe warning and add the unsafe URL to the warning message
* Add test that fails if g is added to the sanitizer
This only affects the prod version since the warning is deduped anyway.
* Fix prod test
* Deprecate ref.setNativeProps in favor of ReactNative.setNativeProps
* Using a feature flag for the setNativeProps warning
* Removing extra line breaks
* Set the FB native feature flag to true
* Prettier
* Turned enableHooks feature flag on everywhere
* Removed useHooks feature flag from tests (now that it's on by default)
* Remove useHooks feature flag entirely
Disables the recently introduced (#14181) warning for shorthand
CSS property collisions by wrapping in a feature flag. Let's hold off
shipping this until at least the next minor.
Removes the `enableDispatchCallback` feature flag and deletes the
associated code. An earlier version of the Hooks proposal included this
feature but we've since decided to remove it.
* Removed the enableGetDerivedStateFromCatch feature flag (aka permanently enabled the feature)
* Forked/copied ReactErrorBoundaries to ReactLegacyErrorBoundaries for testing componentDidCatch
* Updated error boundaries tests to apply to getDerivedStateFromCatch
* Renamed getDerivedStateFromCatch -> getDerivedStateFromError
* Warn if boundary with only componentDidCatch swallows error
* Fixed a subtle reconciliation bug with render phase error boundary
* 🔥 Stop syncing the value attribute on inputs
* Eliminate some additional checks
* Remove initialValue and initialWrapper from wrapperState flow type
* Update tests with new sync logic, reduce some operations
* Update tests, add some caveats for SSR mismatches
* Revert newline change
* Remove unused type
* Call toString to safely type string values
* Add disableInputAttributeSyncing feature flag
Reverts tests to original state, adds attribute sync feature flag,
then moves all affected tests to ReactFire-test.js.
* Revert position of types in toStringValues
* Invert flag on number input blur
* Add clarification why double blur is necessary
* Update ReactFire number cases to be more explicite about blur
* Move comments to reduce diff size
* Add comments to clarify behavior in each branch
* There is no need to assign a different checked behavior in Fire
* Use checked reference
* Format
* Avoid precomputing stringable values
* Revert getToStringValue comment
* Revert placement of undefined in getToStringValue
* Do not eagerly stringify value
* Unify Fire test cases with normal ones
* Revert toString change. Only assign unsynced values when not nully
* Merged interaction-tracking package into react-scheduler
* Add tracking API to FB+www builds
* Added Rollup plugin to strip no-side-effect imports from Rollup bundles
* Re-bundle tracking and scheduling APIs on SECRET_INTERNALS object for UMD build (and provide lazy forwarding methods)
* Added some additional tests and fixtures
* Fixed broken UMD fixture in master (#13512)
* Updated suspense fixture to use new interaction-tracking API
* Integrated Profiler API with interaction-tracking API (and added tests)
* Pass interaction Set (rather than Array) to Profiler onRender callback
* Removed some :any casts for enableInteractionTracking fields in FiberRoot type
* Refactored threadID calculation into a helper method
* Errors thrown by interaction tracking hooks use unhandledError to rethrow more safely.
Reverted try/finally change to ReactTestRendererScheduling
* Added a $FlowFixMe above the FiberRoot :any cast
* Reduce overhead from calling work-started hook
* Remove interaction-tracking wrap() references from unwind work in favor of managing suspense/interaction continuations in the scheduler
* Moved the logic for calling work-started hook from performWorkOnRoot() to renderRoot()
* Add interaction-tracking to bundle externals. Set feature flag to __PROFILE__
* Renamed the freezeInteractionCount flag and replaced one use-case with a method param
* let -> const
* Updated suspense fixture to handle recent API changes
* Removed enableInteractionTrackingObserver as a separate flag; only enableInteractionTracking is used now
* Added interaction-tracking/subscriptions bundle and split tests
* Added multi-subscriber support
* Moved subscriptions behind feature flag
* Fixed bug with wrap() parameters and added test
* Replaced wrap arrow function
**what is the change?:**
Basically undoes 4b2e65d32e (diff-904ceabd8a1e9a07ab1d876d843d62e1)
**why make this change?:**
We rolled out this fix internally and in open source weeks ago, and now
we're cleaning up.
**test plan:**
Ran tests and lint, and really we have been testing this because the
flag is open internally as of last week or so.
**issue:**
Internal task T29948812 has some info.
* add legacy context APIs warning in strict mode
* refactor if statement and the warning message
* add other flags for type check
* add component stack tree and refactor wording
* fix the nits
* Extract base Jest config
This makes it easier to change the source config without affecting the build test config.
* Statically import the host config
This changes react-reconciler to import HostConfig instead of getting it through a function argument.
Rather than start with packages like ReactDOM that want to inline it, I started with React Noop and ensured that *custom* renderers using react-reconciler package still work. To do this, I'm making HostConfig module in the reconciler look at a global variable by default (which, in case of the react-reconciler npm package, ends up being the host config argument in the top-level scope).
This is still very broken.
* Add scaffolding for importing an inlined renderer
* Fix the build
* ES exports for renderer methods
* ES modules for host configs
* Remove closures from the reconciler
* Check each renderer's config with Flow
* Fix uncovered Flow issue
We know nextHydratableInstance doesn't get mutated inside this function, but Flow doesn't so it thinks it may be null.
Help Flow.
* Prettier
* Get rid of enable*Reconciler flags
They are not as useful anymore because for almost all cases (except third party renderers) we *know* whether it supports mutation or persistence.
This refactoring means react-reconciler and react-reconciler/persistent third-party packages now ship the same thing.
Not ideal, but this seems worth how simpler the code becomes. We can later look into addressing it by having a single toggle instead.
* Prettier again
* Fix Flow config creation issue
* Fix imprecise Flow typing
* Revert accidental changes
* Timeout component
Adds Timeout component. If a promise is thrown from inside a Timeout component,
React will suspend the in-progress render from committing. When the promise
resolves, React will retry. If the render is suspended for longer than the
maximum threshold, the Timeout switches to a placeholder state.
The timeout threshold is defined as the minimum of:
- The expiration time of the current render
- The `ms` prop given to each Timeout component in the ancestor path of the
thrown promise.
* Add a test for nested fallbacks
Co-authored-by: Andrew Clark <acdlite@fb.com>
* Resume on promise rejection
React should resume rendering regardless of whether it resolves
or rejects.
* Wrap Suspense code in feature flag
* Children of a Timeout must be strict mode compatible
Async is not required for Suspense, but strict mode is.
* Simplify list of pending work
Some of this was added with "soft expiration" in mind, but now with our revised
model for how soft expiration will work, this isn't necessary.
It would be nice to remove more of this, but I think the list itself is inherent
because we need a way to track the start times, for <Timeout ms={ms} />.
* Only use the Timeout update queue to store promises, not for state
It already worked this way in practice.
* Wrap more Suspense-only paths in the feature flag
* Attach promise listener immediately on suspend
Instead of waiting for commit phase.
* Infer approximate start time using expiration time
* Remove list of pending priority levels
We can replicate almost all the functionality by tracking just five
separate levels: the highest/lowest priority pending levels, the
highest/lowest priority suspended levels, and the lowest pinged level.
We lose a bit of granularity, in that if there are multiple levels of
pending updates, only the first and last ones are known. But in practice
this likely isn't a big deal.
These heuristics are almost entirely isolated to a single module and
can be adjusted later, without API changes, if necessary.
Non-IO-bound work is not affected at all.
* ReactFiberPendingWork -> ReactFiberPendingPriority
* Renaming method names from "pending work" to "pending priority"
* Get rid of SuspenseThenable module
Idk why I thought this was neccessary
* Nits based on Sebastian's feedback
* More naming nits + comments
* Add test for hiding a suspended tree to unblock
* Revert change to expiration time rounding
This means you have to account for the start time approximation
heuristic when writing Suspense tests, but that's going to be
true regardless.
When updating the tests, I also made a fix related to offscreen
priority. We should never timeout inside a hidden tree.
* palceholder -> placeholder
Add a new component type, Profiler, that can be used to collect new render time metrics. Since this is a new, experimental API, it will be exported as React.unstable_Profiler initially.
Most of the functionality for this component has been added behind a feature flag, enableProfileModeMetrics. When the feature flag is disabled, the component will just render its children with no additional behavior. When the flag is enabled, React will also collect timing information and pass it to the onRender function (as described below).
* Remove the 'alwaysUseRequestIdleCallbackPolyfill' feature flag
**what is the change?:**
Removes the feature flag 'alwaysUseRequestIdleCallbackPolyfill', such
that we **always** use the polyfill for requestIdleCallback.
**why make this change?:**
We have been testing this feature flag at 100% for some time internally,
and determined it works better for React than the native implementation.
Looks like RN was overriding the flag to use the native when possible,
but since no RN products are using 'async' mode it should be safe to
switch this flag over for RN as well.
**test plan:**
We have already been testing this internally for some time.
**issue:**
internal task t28128480
* fix mistaken conditional
* Add mocking of rAF, postMessage, and initial test for ReactScheduler
**what is the change?:**
- In all tests where we previously mocked rIC or relied on native
mocking which no longer works, we are now mocking rAF and postMessage.
- Also adds a basic initial test for ReactScheduler.
NOTE -> we do plan to write headless browser tests for ReactScheduler!
This is just an initial test, to verify that it works with the mocked
out browser APIs as expected.
**why make this change?:**
We need to mock out the browser APIs more completely for the new
'ReactScheduler' to work in our tests. Many tests are depending on it,
since it's used at a low level.
By mocking the browser APIs rather than the 'react-scheduler' module, we
enable testing the production bundles. This approach is trading
isolation for accuracy. These tests will be closer to a real use.
**test plan:**
run the tests :)
**issue:**
internal task T28128480
We'll use this in www to test whether the polyfill is better at
scheduling high-pri async work than the native one. My preliminary tests
suggest "yes" but it's hard to say for certain, given how difficult it
is to consistently reproduce the starvation issues we've been seeing.
