This reverts commit 6ea69a0d4339dd153899bb6c299689f1dd43329d.
On top of the revert, it reverted a commit in the PR:
https://github.com/flutter/engine/pull/14024
This reverts commit ea67e5b0b930ebf552bc7dbd678a35ee6a129c39.
This re-enables thread safety checks for WeakPtr. WeakPtrs can't be used on a thread other than the one the WeakPtrFactory was created on.
This fixes the unit tests and adds a getUnsafe() method to WeakPtr to work around the remaining unresolved locations where we are using WeakPtr unsafely.