* chore(ng2/dart): update from beta.1x to beta.18
Update made to all `pubspec.yaml` files and to relevant prose.
Note that pipes was back a beta.15; the rest were at beta.17.
* chore(pipes/dart): update ngFor microsyntax and transform method param
Updates to pipe example source code:
- Pipe `transform()` method parameters
- `ngFor` microsyntax, from `#` to `let`
* 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