Initial work to add Mozart child views

'flutter start' calls 'flutter apk' if necessary.

Fix formatting on AssetVendor's dart doc

flutter start no longer depends on a pre-built SkyShell.apk. It builds a
new one, as long as an AndroidManifest.xml exists.

We rebuild the .apk every time either AndroidManifest.xml or
flutter.yaml changes.

normalize -t --target option

Move ServiceMocker into flutter_test

People writing their own tests will want to mock services as well.

Update flutter_sprites now that ImageResource.first returns an ImageInfo

Actually return a SemanticsServer service.

add docs for -h -v

Rename "Toggle Controls" to "Selection Controls"

This patch sketches out the basic widgets for creating Mozart child
views. We're still missing some of the machinery, so we can't quite
create child views yet, but once we supply the ViewHost and teach the
compositor how to actually display the child scenes, we'll have
something that works.

Fixes #1537

RawKeyboardListener asserts if disposed without keyboard

We need to check whether we're attached to the keyboard before trying to
detach from the keyboard.

AppBar extensions for Scaffold et al

Add support for the appbar behavior described in the "Flexible space with image" section of https://www.google.com/design/spec/patterns/scrolling-techniques.html#scrolling-techniques-scrolling.

For now, an "AppBar" is a toolbar, a toolbar and a tabbar arranged in a column, or a toolbar and flexible space widget arranged in a stack with the toolbar on top. The AppBar occupies the space behind the status bar, i.e. its origin is at the top of the screen.

The Scaffold listens for scroll notifications from a Scrollable descendant identified by scrollableKey. Scrolling downwards from the top causes the Scaffold to shrink its appBar's height from appBarHeight to 0. Scrolling downwards when the appBar isn't visible causes its toolbar to reappear. Applications can specify the flexibleSpace - essentially the appbar's background - with a WidgetBuilder. The contents of the flexible space can track the appbar's relative height with Scaffold.of(context).appBarAnimation, an animation that's 0.0 when the appbar is fully visible and 1.0 when it has disappeared.

Added FlexibleSpaceBar to simplify building a flexible space widget that animates a title and a background image per the Material Design spec.

To enable the new behavior, specify the following Scaffold properties:

```
return new Scaffold(
  appBarHeight: appBarHeight,
  scrollableKey: scrollableKey,
  appBarBehavior: AppBarBehavior.scroll,
  ...
)
```

AppBarHeight should be greater than kToolBarHeight. Typically it will be much greater.

ScrollableKey identifies the Scrollable that the Scaffold will track.

AppBarBehavior.scroll indicates that the appbar will resize in response to scrolling.

The Scrollable must be a descendant of the Scaffold body and must specify the same scrollableKey as well as a padding value that accounts for the appbar's height. For example:

```
  body: new ScrollableViewport(
    key: scrollableKey,
    child: new Padding(padding: new EdgeDims.only(top: appBarHeight)
    ...
   )
```

Also:
- Added scrollableKey and scrollablePadding to MaterialList. They're applied to the ScrollableList child.
- Fixed the padding support in ScrollableList.
- Added a foregroundAlpha property to ToolBar. It controls the opacity of the icon and text themes' colors.
- Removed the toolbar's "bottom" widget property. Use the flexibleSpace WidgetBuilder instead.

Add support for the appbar behavior described in the "Flexible space with image" section of https://www.google.com/design/spec/patterns/scrolling-techniques.html#scrolling-techniques-scrolling.

Viewport fails to clip its AssetImage child

We were getting confused about our layer tree offsets, which caused us
to set an incorrect cull rect for the recording inside a viewport.

This patch does away with layer tree offsets almost entirely. We now use
them only at repaint boundaries, which is where we want the layer tree
to be mutable.

Fixes #1460

tell people to submit code after LGTM

Add scale awareness to images

Update to new editing.mojom

add --start-paused,--debug-port flags

add section on submitting code in CONTRIBUTING.md

add debugFillDescription to OverflowBox

Now the keyboard is responsible for maintaining the state of the text
field.

In addition to checking for the existence of the engine, ensure that it’s the correct version