To make the units clear.

Make sure to send tapcancel when the primary pointer fails because of
slop, even if the gesture won by default.

Also, minor cleanup and clarification of an invariant.

Draggable is now itself a gesture arena member. This means it won't
conflict with other gesture recognisers in the same path.

This also allows variants of Draggable that are triggered by other
gestures.

Also, some cleanup of DoubleTapGestureRecognizer, GestureDetector, and
PrimaryPointerGestureRecognizer.

Also, make MultiTapGestureRecognizer support a timeout for longpress.

Also, make Draggable data be typed.

Also, hide warnings about constructor warnings for now. Analyzer doesn't
support them yet. (Have to do this on a per-line basis)

Directions for future research:
 - animating the avatar (enter/exit transitions)
 - interaction with the navigator (canceling a drag on page navigation, etc)
 - double-tap draggable

Make all the *GestureRecognizer classes inherit from a class called
GestureRecognizer. Give the old GestureRecognizer a name that is more
precise about its purpose. Remove the members of GestureArenaMember that
aren't used by GestureArenas.

Document the Gesture Detector constants.

Remove kEdgeSlop since it's obsolete on Android.

Add a test that verifies that when the first tap is canceled, the second
tap can become the first tap of a subsequent two-tap sequence.

Tap and the othes classes in tap.dart are not really related.

Turns out there were no uses of GestureArenaMember other than those that
use DisposableArenaMember.

Fixes #1807

Fixes #1807

We don't want to expose these from the gesture package. This patch moves
DoubleTapGestureRecognizer into tap.dart so we can make TapTracker and
TapGesture private to that file.

Don't use a timeout to cancel tap tracking. Track only one primary pointer
and ignore non-primary pointers. Update tests to reflect desired behaviors.

Fixes #1779, #1780, #1781.

Now we use one velocity tracker per pointer.

Path by Kris Giesing.

Fixes #1704

They should be 0.0 when there's no offset.

Fixes #1661

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.

After this patch, InkWell is driven by gesture recognizers, which lets us
cleanly cancel splashes when the user actually scrolls.

I've also refactored all the clients of InkWell to use InkWell to detect
gestures instead of wrapping InkWell in a GestureDetector.

Fixes #1271

 * Rename GestureTapListener (and friends) To GestureTapCallback to match the
   other gesture callbacks.
 * Replace "ensureFoo" pattern with ??= operator.

Fixes #1320