* working on making a faster flutter run restart

* clean up todos; fire events on isolate changes

* use the Flutter.FrameworkInitialization event

* review comments

This reverts commit 8f69cccc.

* dissallow -release mode on emulators

* have the help text print the default mode

This passes the information through Xcode to the `flutter build aot` call via
the environment so that it picks up the right artifacts.

Fixes #4186

* handle an exit code from android n

* review comments

* rename service_protocol.dart to protocol_discovery.dart

* add a wrapper around the obs. protocol

* use json-rpc in run

* consolidate obs. code; implement flutter run --benchmark

* review comments

* use vendored dart sdk; check the expected version of flutter

* search up directories looking for the flutter root

* send ext.flutter.exit

* listen for help restart, quit

* refactor into a separate class

* avoid sending analytics on ci systems

* review comments

* fix analysis errors

* review comments; fix test

* re-add an export for debugPrint

* rework flutter run

* fix npe with --debug-port

* connect to obs and exit when that conneciton closes

* update todos

* add a service extension for repaint rainbow

* review comments

Handy for testing against specific local SDK builds.

(Note that the option is hidden.)

* download android-arm-deploy

* wire up --deploy to android-arm-deploy

* fix interpolation

* use the new cache code to locate more artifacts

* add todo

* review comments

* more terse flutter upgrade

* fix an issue when updating and the engine update code has changed

* call flutter precache; add a --no-color hidden option

* fix a lint related to getters/setters

* better messaging about windows support

* fix lints

* move the engine download code to dart

* fix populating the bin/cache/pkg dir

Add a command that can capture a Skia picture of a Flutter frame and send it to a file or skiaserve debugger (#3165)

* use --no-precompile

* use the flutter bin cache's sdk

* add cache.dart to help manage the cache dir

* sp

flutter start's method of finding devices to run the app on is not suitable for flutter drive.

This commit also refactors several tool services to allow mocking in unit tests.

"flutter create" adds option `--with-driver-test` that adds
dependencies to `flutter_driver` in `pubspec.yaml` and creates
a basic driver test runnable via `flutter drive
--target=test_driver/e2e.dart`

"flutter drive" new options:

- `--keep-app-running` tells the driver to not stop the app after tests
  are done
- `--use-existing-app` tells the driver to not start a new app but use
  an already running instance