Commits on Sep 8, 2020

1. Decouple public, internal act implementation …

   In the next major release, we intend to drop support for using the `act`
   testing helper in production. (It already fires a warning.) The
   rationale is that, in order for `act` to work, you must either mock the
   testing environment or add extra logic at runtime. Mocking the testing
   environment isn't ideal because it requires extra set up for the user.
   Extra logic at runtime is fine only in development mode — we don't want
   to slow down the production builds.
   
   Since most people only run their tests in development mode, dropping
   support for production should be fine; if there's demand, we can add it
   back later using a special testing build that is identical to the
   production build except for the additional testing logic.
   
   One blocker for removing production support is that we currently use
   `act` to test React itself. We must test React in both development and
   production modes.
   
   So, the solution is to fork `act` into separate public and
   internal implementations:
   
   - *public implementation of `act`* – exposed to users, only works in
     development mode, uses special runtime logic, does not support partial
     rendering
   - *internal implementation of `act`* – private, works in both
     development and productionm modes, only used by the React Core test
     suite, uses no special runtime logic, supports partial rendering (i.e.
     `toFlushAndYieldThrough`)
   
   The internal implementation should mostly match the public
   implementation's behavior, but since it's a private API, it doesn't have
   to match exactly. It works by mocking the test environment: it uses a
   mock build of Scheduler to flush rendering tasks, and Jest's mock timers
   to flush Suspense placeholders.
   
   ---
   
   In this first commit, I've added the internal forks of `act` and
   migrated our tests to use them. The public `act` implementation is
   unaffected for now; I will leave refactoring/clean-up for a later step.

   

   acdlite committed Sep 8, 2020
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