Add type annotations in many places.
Fix some identifiers to have more lint-satisfying names.
Make all operator==s consistent in style.
Reorder some functions for consistency.
Make ParentData no longer dynamic, and fix all the code around that.

Now that you import a top-level file for each layer of the Flutter
framework, within the framework we can use relative paths without being
worried about the copy/pasta problem we used to have.

Fixes #1320

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.