Fixes https://github.com/flutter/flutter/issues/8476
More detailed list of changes in this patch:
* Replaced the didTouch special logic with more generic logic that
uses Activities instead. Now instead when you tap down the
Scrollable calls `hold()` which begins a HoldScrollActivity which is
a hybrid of DragStartDetails and IdleScrollActivity and can be
canceled. When you let go, it gets canceled and that goes ballistic.
* Make DragGestureRecognizer more aggressive about grabbing pointers,
otherwise a second pointer in a situation with competing horizontal
and vertical recognizers always gets taken by the other one.
* Fixed the _GestureSemantics widget to call the "down" callbacks so
that it follows the same pattern as "real" interactions.
* Added tests for the above.
* Added a hashCode to ScrollActivity.toString (and subclasses).
* Added a toString to ScrollDragController, and include it in
DragScrollActivity's toString.
* s/coorindator/coordinator/
* Add a comment in DragStartDetails to distinguish it from the
otherwise identical DragDownDetails, so we're not tempted to merge
them.
f64bfba860