Fixes https://github.com/flutter/flutter/issues/5380

Because parent structure changes when slices gets separated and
merged, children widgets can be rebuilt redundantly. This commit
adds a global key to each child so that the framework always knows
its children apart.

Fixes: https://github.com/flutter/flutter/issues/5336

Fixes https://github.com/flutter/flutter/issues/5131

* Add Dart SDK to version info (#4986).

Adds a new SDK line to the `flutter —version` output.

* Prefer `Platform.version`.

* Version text fixes.

MergeableMaterial is an animated container that knows how to merge
separate slices of Material together.

Added a simple widget that automatically dispatches a
LayoutChangedNotification when its child changes layout.

Added a flag that instructs Stack how to deal with overflowing
children: they can either be clipped or not.

- [x] Refactor view support into separate classes.
- [x] Make the hot runner grab the main view and call runFromSource on it.
- [x] Remove Device.needsDevFS (because it is always true).

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.