This patch is part of a sequence of patches towards fewer top-level libraries.
In this patch, the gesture libraries are combined into one gestures.dart
library.

That way the fling engages in the same direction as the scroll. For example, if
you have a horizontal scroll nested inside a vertical scroll, the fling will
take place in the same direction as the scroll.

These files really belong on other libraries.

If there are no other gestures in the arena, we should kick off the scroll
gesture right away. This change pulled a refactoring of how we dispatch events
to Widgets. Now we dispatch events to Widgets interleaved with their associated
RenderObjects. (Previously we dispatched to all of the RenderObjects first.)

This patch replaces the scroll gestures used by Scrollable with ones detected
by the ScrollGestureDetector.

These gestures use the new gesture detection system.