Flutter widget tests assert if a test completes with timers still
pending. However, it can be hard to diagnose where a pending timer
came from. For example, a widget might consume a third-party library
that internally uses a timer.
I added a FakeAsync.pendingTimersDebugInfo getter to quiver
(https://github.com/google/quiver-dart/pull/500). Make flutter_test
use it.
Additionally modify Flutter's debugPrintStack to take an optional
StackTrace argument instead of always printing StackTrace.current.
Fixes#4237.
b2f8d3a668