Refactor PaintingContext

This patch simplifies PaintingContext with several goals:

1) We now call a callback instead of assuming the caller has a single child to
   paint. This change will let us handle render objects that wish to paint more
   than one child.
2) We now avoid creating lots of empty picture layers because we don't eagerly
   start recording pictures. Instead, we wait for the first caller to touch the
   canvas before creating the picture recorder.
3) We now are more consistent about which values have incorporated the painting
   offset.

Point to Github issues instead of flutter.io

Fixes #489.

@sethladd

Popup menu in Stocks throws exception

Previously, we were putting a ForcedLayer just below the overlay, but that
causes trouble for routes like the popup menu that want to position themselves
inside the overlay. Instead, I've moved the ForcedLayer down into ModalRoute.

Also, rename ForcedLayer to RepaintBoundary, which is more descriptive of what
this widget does.

Fixes #485

Add `alignment` to image widgets

We already had the code to implement this feature. We just needed to plumb it
out to the widget API.

Snackbar Refactor

"showSnackBar()" is now a feature of a Scaffold. To get to a Scaffold
you either use a global key (`scaffoldKey.currentState.showSnackBar(...)`),
or you use `Scaffold.of(context)`.

Snack bars no longer have a route. They are entirely managed by the
Scaffold. Fixes #432.

Snack bars now queue up when you have several of them. Fixes #374.

Snack bars now auto-size themselves around their contents. This is step
one towards implementing multiline snack bars.

Snack bars now self-dismiss after some per-snackbar configurable period.

The self-dismissing pauses while a dialog is up above the snackbar (or
anything that uses ModalRoute). To enable this, there's now a
`ModalRoute.of(context)` API that returns the current ModalRoute, and
you will be rebuilt if you asked for this and the route's "current"
status changes. To implement this, the Navigator now rebuilds
unconditionally any time it pushes or pops a route.

Snack bars now use the curves that Android uses for snack bars.

Snack bar contents now fade in.

Reduce paint time for Stocks drawer animation by 88%

Now we use a forced layer around the reprojected content of the drawer, which
means we don't have to re-record it during the slide animation. This saves 2ms
per frame.

The total main-thread time for the drawer animation is now 2.0ms.

Reduce record time in Stocks drawer animation by 55%

Now we use a ForcedLayer in the navigator overlay to cache the recording for
each entry in the overlay. This mechanism just caches the display list, not the
underlying pixels.

Also, remove the "dispose" notification in the Layer tree because it was
disposing layer too eagerly. We don't actually need this notification for
anything other than eagerly freeing some C++ memory.

Corrected Card resting elevation, tabs example toolbar

Fixes #411

Switch from ui.tracing to dart:Developer Timeline

This change will let us remove the tracing interface from dart:ui and also
makes these timeline events show up in Observatory.

Update Material shadow rendering

Shadows now render as three seprate MaskFilter.blur components per the most recent Material spec.

The shadows Map was replaced by a similar Map called elevationToShadow with entries that match the 10 elevations specifed by http://www.google.com/design/spec/what-is-material/elevation-shadows.html.

The "level" property (many classes) is now called "elevation", to match the Material spec.

BoxShadow now includes a spreadRadius parameter - as in CSS box-shadow. Renamed the BoxShadow blur property to blurRadius to further align BoxShadow with CSS box-shadow.

Handle adb error cases more gracefully

Shadows now render as three seprate MaskFilter.blur components per the most recent Material spec.

The shadows Map was replaced by a similar Map called elevationToShadow with entries that match the 10 elevations specifed by http://www.google.com/design/spec/what-is-material/elevation-shadows.html.

The "level" property (many classes) is now called "elevation", to match the Material spec.

BoxShadow now includes a spreadRadius parameter - as in CSS box-shadow. Renamed the BoxShadow blur property to blurRadius to further align BoxShadow with CSS box-shadow.

We now print a sensible message if we can't find `dart` or `adb`. Also, we
print a sensible message if the device isn't authorized.

Fixes #380
Fixes #358

Improve error message for non-existent asset

Import path.dart as path rather than p

Now we print the name of the asset and the base directory where we looked for
the error instead of throwing a null pointer exception.

Fixes #296

Fixes #311

Add support for repeating performances

Pave the Foo.of() cowpath.

Make flutter run_mojo imply flutter build

Fixes #447

I'm planning on adding more .of() functions and so to avoid further
copypasta I'm providing some general utility functions here.

Rename StatisticsBox to RenderStatisticsBox

This makes run_mojo more similar to flutter start.

Fixes #441

Add a task scheduler.

Improve flutter --version output

The current implementation is still pretty rudimentary.

Fix focus bug that was breaking nested navigators

Turns out we were assuming that once the global key is gone, our
listener is unregistered. It isn't.

I'm planning on adding a test as part of my nested navigator work.

Add a Flutter command that builds an APK using a local build of the e…