Callback identity is too fragile for CustomPaint

Many of the widgets that use CustomPaint were spamming repaints because
CustomPaint repaints when the identity of the onPaint callback changes, which
it does every build for StatelessComponents.

This patch changes CustomPaint to use a CustomPainter, similar to the new
custom layout widgets. The CustomPainter has a `shouldRepaint` function along
with its `paint` function. This function gives clients explicit control over
when the custom paint object repaints.

Add a FractionalOffset alignment parameter to BackgroundImage

Add a repaint rainbow to help debug repaint issues

The repaint raindbow tints each recording with a rotating set of colors so you
can see which pixels are repainting each frame.

Add debugDumpLayerTree to dump the layer tree

To help debugging issues with the layer tree.

Re-enable support for debugPaintLayerBordersEnabled

Removes demo game from repository (it's now in its own repo)

We lost this feature at some point. This patch brings it back.

Introduced ImageProvider for asynchronously loading images.

Some paint functions were using canvas.save/restore around children

Updated image cache to use any ImageProvider instance.
Renamed RawImage to AsyncImage and updated the interface
to use any ImageProvider instance.

This pattern breaks when using compositing because we need to lift those
operations into the compositing tree. This patch removes all the ones I could
find and adds an assert to help prevent more from getting introduced.

Fixes #191

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