Also:
- add operator==/hashCode/toString to ViewportDimensions
- add toString to BindingBase
- add toString and debugFillDescription to ScrollBehavior
- fix a bug in the RawGestureDetectorState's replaceGestureRecognizers
- rename MixedViewport's onExtentsUpdate to onExtentChanged
- replace ExtentsUpdateCallback with ValueChanged<double>
- remove a microtask for dispatching scroll start, since it
did not appear to have any purpose
- added dartdocs to Instrumentation until I understood it
- made all event dispatch in Instrumentation drain microtasks
f808055756