Commits on May 2, 2020

1. Fix tests that only notify most recent listener …

   These tests were written in such a way that when a wakeable has multiple
   listeners, only the most recent listener is notified.
   
   I switched them to use a promise instead of mutating a ref.

   

   acdlite committed May 2, 2020
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2. Initial Lanes implementation …

   A refactor of our concurrency and scheduling architecture. Replaces
   the ExpirationTime type with a new concept, called Lanes.
   
   See PR facebook#18796 for more information.
   
   All of the changes I've made in this commit are behind the
   `enableNewReconciler` flag. Merging this to master will not affect the
   open source builds or the build that we ship to Facebook.
   
   The only build that is affected is the `ReactDOMForked` build, which is
   deployed to Facebook **behind an experimental flag (currently disabled
   for all users)**. We will use this flag to gradually roll out the new
   reconciler, and quickly roll it back if we find any problems.
   
   Because we have those protections in place, what I'm aiming for with
   this initial PR is the **smallest possible atomic change that lands
   cleanly and doesn't rely on too many hacks**. The goal has not been to
   get every single test or feature passing, and it definitely is not to
   implement all the features that we intend to build on top of the new
   model. When possible, I have chosen to preserve existing semantics and
   defer changes to follow-up steps. (Listed in the section below.)
   
   (I did not end up having to disable any tests, although if I had, that
   should not have necessarily been a merge blocker.)
   
   For example, even though one of the primary goals of this project is to
   improve our model for parallel Suspense transitions, in this initial
   implementation, I have chosen to keep the same core heuristics for
   sequencing and flushing that existed in the ExpirationTimes model: low
   priority updates cannot finish without also finishing high priority
   ones.
   
   Despite all these precautions, **because the scope of this refactor is
   inherently large, I do expect we will find regressions.** The flip side
   is that I also expect the new model to improve the stability of the
   codebase and make it easier to fix bugs when they arise.

   

   acdlite committed May 2, 2020
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