Add reference to "mojo:media_service" in sound.dart

Related: https://github.com/domokit/mojo/issues/614

Flutter can play sounds using this package. However, when Mojo Shell is running Flutter, it lacks the necessary media service.

This patch creates a hook to the mojo binary `media_service.mojo`. If Mojo Shell is built with this media service, then it will be able to play sounds when running Flutter.

RenderPerformanceOverlay needs alwaysNeedsCompositing

The performance overlay is always drawn using the compositor. We should tell
the rest of the system that it's going to make a composited layer.

Fixes #1177

Use a named argument for Widget children

Previously we used a positional argument for widgets that had multiple
children. Now we use a named argument that defaults to an empty list.

Fixes #241

rev engine

Rename StatisticsOverlay to PerformanceOverlay.

Rename ScrollDirection to Axis

We use the ScrollDirection for more than just scrolling.

Fixes #151

Adds Flutter GCM package

Always using OpacityLayer for blending

We don't know how to accuately compute paint bounds in the render tree.
Instead, we can rely on the compositor to compute the paint bounds for
us if we use OpacityLayer to do our opacity blends.

Fixes the shadow when closing the menu in the stocks app.

Rename RenderObject.hasLayer to isRepaintBoundary

Also, introduce alwaysNeedsCompositing to force the "needs compositing"
bit to true without necessarily introducing a ContainerLayer into the
layer tree. A future patch will make use of alwaysNeedsCompositing to
optimize opacity layers.

DoubleTap gesture asserts when rejected

The pointer router was using an iteration pattern that always delivers
handleEvent calls even if you remove a route during the iteration.
That's awkward to program against and causes trouble for the double-tap
gesture.

This patch switches PointerRouter to using a re-entrant iteration
pattern that supports removing routes (but not adding routes) during the
iteration.

Add more debugging options to the stocks app.

TabView fails to update tab contents

- Add an option to enable debugPaintSizeEnabled.
- Add an option to enable the StatisticsOverlay.
- Add support for the StatisticsOverlay in MaterialApp.
- Change the layout behaviour of RenderStatisticsBox:
   - Give it a zero intrinsic width.
   - Give it an accurate intrinsic height that depends on the flags
     set. (Also, move the enum to the rendering/ layer from the
     widgets/ layer to enable this.)
   - Make the box automatically size itself full-width and the correct
     height, so that you can actually embed it (though most of the
     time you'd just put it in a Stack so this doesn't matter as
     much, really).
- Some style nit fixes in statistics_box.dart.

When the TabView widget's children change, we don't change the children
we give to the PageViewport because we fail to update _items. This patch
watches didChangeConfig and updates _items appropriately.

Fixes #1134
Fixes #1168

Simplify TabView API

Instead of taking a builder and a generic type, we can just take a
List<Widget>.

Merge Container's width, height, and constraints

Without this, FlatButton ends up using two RenderConstrainedBoxes back
to back for no good reason.

tightenWidth(), tightenHeight() => tighten(width:, height:)

Add a note about the firebase/ directory.

This makes it more consistent with tightFor(), and also makes it
easier to tighten both directions at once when you're not sure you
will always do so (e.g. if you have a height and width that might be
null, and want to tighten whichever ones aren't null).

Also, make dev/update_packages.dart skip directories that have no
pubspec.yaml, since firebase/ doesn't have one.

AnimatedPositioned

This will allow AnimatedPositioned to reuse all the same logic.

Drawer edge swipe convers entire screen

Refactor the Stocks app's settings

This will make it much easier to add more settings.