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

Double-tapping modal barrier pops twice

_ModalScope has local copies of route state

Now we just use the state from the Route.

Fixes #744

Make BoxDecoration replaceable.

Factor out a reusable interface called Decoration from BoxDecoration.

Make all the consumers of BoxDecoration and the erstwhile BoxPainter
into consumers of Decoration.

Make a BoxPainter be something you get from a Decoration, rather than
something to which you pass a BoxDecoration.

Rename Shape to BoxShape now that it's documented specifically as
applying to boxes.

Move EdgeDims to its own file.

Move FractionalOffset up so that it's with the other helper classes in
its file rather than alone at the end.

Minor change to RenderClipOval's hit testing to avoid taking an
unnecessary square root.

Rename BoxDecorationPosition to DecorationPosition since
RenderDecoratedBox now takes any Decoration.

Implement hit testing for rounded rects.

Rename AnimatedBoxDecorationValue to AnimatedDecorationValue, and make
it support lerping across any Decoration (by deferring to the objects
involved).

Add a LocaleQuery widget that can be used to fetch locale-specific data

Users of MaterialApp can provide an onLocaleChanged handler that will be
called to asynchronously fetch locale-specific data.  MaterialApp will
then instantiate a LocaleQuery that supplies the locale data to its
descendants.

Now we ignore the pointer after we start popping the route.

Fixes #686

Material gallery crashes when you press the drop-down button

Now use use the route's getPosition function to position the drop-down menu.
Also, fix a number of other related bugs that blocked the dropdown button from
working correctly. The dropdown menu still has the following issues:

1) In the exit animation, the background of the menu disappears too quickly
   because of incorrect paint bounds computations in the layer tree.
2) The drop down menu isn't positioned correctly after the device rotates.
   We'll need to address this issue in a separate patch.

Fixes #630

didChangeNext()

Replace didPushNext() and didReplaceNext() with didChangeNext(), and
call it in more cases, so that a route can easily track the next route.

Use this to make TransitionRoute properly track its next route so that
you can do next-route-driven animations that work even with removes,
replaces, and other crazy manipulations of the navigator stack.