Reland: Increase threshold for usage of compute for utf8 decoding on large strings to 50 KB (#64498)

* fix bug when tap ListTitle when tristate enable #63846

* fix bug when tap ListTitle when tristate enable #63846

* code style

* improve the unit test case

* bow to convention

This reverts commit 8b52e6a8.

* Add GestureDetector.onDoubleTapDown()

* Review comments

Removes more mockito to unblock migration of all tests to sound null safety. #62886

Revert "Increase threshold for usage of compute for utf8 decoding on large strings to 50 KB (#64350)" (#64372)

This reverts commit f07f4120.

Reland "Prevent material switch from recreating its render object when it becomes disabled (#61398)", reverted in #64062 (#64354)

On a local benchmark, removing compute is faster than compute in both debug and release mode on a Pixel 4. On a MotoG4, a much larger text sample (800 Kb) takes 50 ms, so we cannot simple remove the limit. Increase to 50 Kb which should take at most a ms or two on an older device, and only microseconds on newerones.

Bug:
  https://github.com/flutter/flutter/issues/64070

More work towards removal of mockito to facilitate strong null safety tests.

#62886

The remaining uses of this pattern were all due to wanting to have
the child's slot when `Element.forgetChild()` was called. However,
when that method is called, the child's `slot` value is still valid
in the context of the parent, so the uses can just use `child.slot`.

This is the final round of cleanup from the fallout of #63269