Changes in this patch:
- iOS now uses a different scrollDrag constant than Android.
   - ScrollConfigurationDelegate now knows about target platforms.
   - ScrollBehaviors now know about target platforms.
   - RawInputLine now has to be told what platform it's targetting.
   - PageableList now has a concept of target platform.
- make debugPrintStack filter its stack.
   - move debugPrintStack to `assertions.dart`.
- add support for limiting the number of frames to debugPrintStack.
- make defaultTargetPlatform default to android in test environments.
- remove OverscrollStyle and MaterialApp's overscrollStyle argument. You
  can now control the overscroll style using Theme.platform.
- the default scroll configuration is now private to avoid people
  relying on the defaultTargetPlatform getter in their subclasses (since
  they really should use Theme.of(context).platform).
- fix some typos I noticed in some tests.
- added a test for flinging scrollables, that checks that the behavior
  differs on the two target platforms.
- made flingFrom and fling in the test API pump the frames.
- added more docs to the test API.
- made the TestAsyncUtils.guard() method report uncaught errors to help
  debug errors when using that API.

* send the base uri back to debuggers

* add a fullRestart parameter to app.restart

* add await

- [x] Update engine to bring in new snapshotter.
- [x] Use the new snapshotter to quickly determine the minimal set of files necessary to run.
- [x] On first DevFS sync, only sync files necessary to run the application.
- [x] Fix a DevFS unit test failure.
- [x] Include DevFS tests in all.dart.

Previously the FLX builder compressed assets only if they were not dynamically
generated.  This meant that the license file was not compressed.

Also, add a "flutter build flx --report-licensed-packages" option for
when you need to get the list of the packages affected by licenses.

This can happen if you build an FLX in release mode for an app with no assets
(such as the hello_world example)

* make flutter run work with a pre-built apk

* refactor to remove the buildDir param

...so that you can use hot reload mode to update assets.

* Added return value to a onNotification callback.

The LayoutChangedNotification callback was missing a return value.
This commit changes it to return true and stop notification from
bubbling up the tree.

* Changed _RenderInkFeatures to use fresh clip box.

Since it wasn't using the most current value of the RenderBox's
size, _RenderInkFeatures was rendering splashes incorrectly when
the underlying Material size was animating. This commit changes
the clip reference to use the size of the Renderbox instead of
the size method in order to get the newest value.

* Shrine tile height now hardcoded to match actual card size.
* Animation demo now scales with screen size.

BUG=https://github.com/flutter/flutter/issues/5002
BUG=https://github.com/flutter/flutter/issues/5003

* support hot mode from the daemon protocol

* move method params to constructor

This reduces dependence on machine-specific font configuration, making
tests a bit more reproducible.

* Fix covariant overrides in SynchronousFuture.

There were two things going on here. In timeout(), the callback's return
type was needlessly tightened to only allow callbacks that return
futures. This makes SynchronousFuture not substitutable with Future,
whose timeout() allows callbacks that return immediate values.

Since SynchronousFuture.timeout() never calls the callback anyway, I
just loosened it to match Future.timeout().

SynchronousFuture.whenComplete() is just wrong. The type error, again,
is that the callback's return type is too tight. Future.whenComplete()
allows synchronous callbacks.

But the actual implementation is wrong as well. whenComplete() should
return a future that completes to the *original value*, not whatever the
callback returns.

So I just fixed the method to work correctly, including handling
callbacks with synchronous results.

* "(error, stackTrace)" -> "(e, stack)".

* Update examples to lower the deployment target on iOS.