Do not throw when resolving a local engine if the .packages map does not include the flutter package (#5519)

Fixes https://github.com/flutter/flutter/issues/5468

This will ensure cleanup of any files that existed in previous versions of
the artifacts but have since been removed

* use transparent android launcher icons

* revert stocks icons

* Build sky_engine Dart SDK summary bundle on FlutterEngine.download().

* Tweaks for Devon comments.

* Tweaks for Phil comments.

Also update the AOT builder for changes to the layout of the sky_engine package

* fix an issue where --hot would not exit

* add type annotations

* update the cache earlier

* add documentation

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

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

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

* Prefer `Platform.version`.

* Version text fixes.

- [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).

* 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)