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…

Sample output:

```
Flutter
Repository: git@github.com:flutter/flutter.git
Branch: master
Revision: 7be58b1a (62 minutes ago)
```

Fixes #433

Example:
   cd flutter/examples/stocks
   flutter --engine-src-path /path/to/engine/src apk -o Stocks.apk -m apk/AndroidManifest.xml

Switch flutter run_mojo over to flutter.mojo

The new flutter.mojo is ready for use.

Notify the previous route when pushing and popping

This makes HeroController ignore StateRoutes when deciding where to
animate heroes to and from.

Convert PointerInputEvent to Duration for timestamp

Allowing us to have a single error seems silly.

To make the units clear.

Fix some examples.

Add a Positioned constructor from a Rect

A common pattern is to use a Positioned with a Sized box to give both an offset
from the edge as well as a fixed size to the child. This patch rolls into into
the Stack layout algorithm for simplicity.

Fixes #250

Add all the icon sizes to the FLX

Add a --version option to flutter

Adding all the sizes of all the icons adds about 50 KB to the stocks FLX.
That's probably the right trade-off until we get better at pruning the set of
assets.

Fixes #235