Cleanup the global scope a bit:

- Remove unused FocusChanged typedef.
- Remove unused centerOfAttentionHeroTag.
- Modernize static functions for interacting with Scrollable by moving them
  into the Scrollable class.

Make the StockHome TabBar selection persistent

Fixes #671

Update documentation based on comments in previous patch

Add some more dartdoc to widgets

Improve tolerance for ending scroll animations

We had the units wrong on the tolerances. Previously we multiplied by the
device pixel ratio, which meant we got larger tolerances as we got more
resolution. Also, simplify logic in Newton for applying the tolerances.

Fixes #828

Catch un-normalized BoxConstraints

Add BoxConstraints.isNormalized feature.

Use this feature in asserts in all the intrinsic dimension methods, in
various relevant BoxConstraints methods, and in layout().

Wrap the _DebugSize logic in BoxConstraints.constrain() in an assert
block to avoid a branch in release mode.

Remove the logic in BoxConstraints.isSatisfiedBy() that dealt with
non-normalized values.

Add BoxConstraints.normalize().

Make RenderCustomOneChildLayoutBox.performLayout() only set
parentUsesSize on the child if the constraints aren't tight.

Update testing instructions per abarths' feedback

Add some dartdoc for GestureDetector

... just a start.

Add a test to stocks for changing the locale

I had to add a setLocale method to WidgetTester and
split the code in FlutterBinding which handled locale
changes to allow me to dispatch a locale change w/o actually
changing what the c++ code reports as the locale.

Also added the test to Travis.

@abarth @jason-simmons

Switch over to the Travis-built artifacts

Make AssetImage constructor const

We've been bulding these binaries using Travis for a while. We can start using
them now.

use async / await

https://github.com/flutter/flutter/commit/724388d24f5f1f9ea2ddc7571f62da68ea776d0f

@abarth

Widgets that depend on Material should assert that

Remove unused EventRecorder

Fix hero transition when using dynamic routes

Clean up pointer handling.

rev engine

After this patch, if you try to use a widget that depends on being enclosed in
a material, you now get an assert and a debugPrint if you're not inside a
material.

Fixes #243

Previously we tried to work around some possible badness from the engine
side, but now we are more assertive about the engine needing to do the
right thing.

Change flutter_sprites to use the current PointerEvent.position API