* 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.
* docs(toh-6/dart): first edition of prose and example code
NOTE: this PR depends on #1686.
Dart prose and example match TS except that:
- No child-to-parent event emission occurs.
- Support for Add Hero is added as an unconditional feature of the
Heroes view.
- http `_post` takes only a name
- http `delete` takes only a hero id.
- The Dart in-memory-data-service has been dropped in favor of an
implementation based on the "standard" `http.testing.MockClient` class.
* post-review changes
Patrice Chalin