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

This option just prints the current git revision.

Fixes #40

Remove redundant fetch functions and avoid crashing when no headers are set

Allow flutter engine path to be set when using run_mojo

Fix issue with forward-delete in editable text widget

Now that #371 is fixed, we no longer need to check logs for "sky"

- drag and drop was using the wrong draggables (long press vs short
  press).

- navigation.dart wasn't using material correctly and so was ending up
  with some red text.

- piano wasn't followig our style guide even a little.

- date picker was not scrollable.

Moar shebang.

Make it possible to run dev/update_packages.dart directly from the shell.

@abarth

Support for setting HTTP headers

Fix #373 Add abbreviation for flutter logs -c