* cache/quickstart sync before making Dart edits
* quickstart updates for Dart
* cache/guide/index sync before making Dart edits
* guide/index updates for Dart
* learning-angular for Dart
* guide/setup for Dart
* refresh cache/tutorial/index
* update to beta.22
* Dart SDK 1.19.0 or later is required
* minor edits to sample app titles to e2e tests pass
* Renamed “Angular 2” to “Angular” so that shared e2e tests pass.
* Tweak to QS prose.
- Renamed `angular2_getting_started` to `angular2_quickstart` (to match
TS).
- s/-/_/g in project names elsewhere (only toh-5 & -6 needed to be
updated).
Fixes#378.
