Contributes to #2407
- Dropped the “2” in “Angular 2” and “angular2_*” where appropriate.
- Did a partial sync of `_cache/guide/pipes.jade`
- In quickstart, changed `Try changing the message to "Hello Angular
2!”` to `Try changing the message to "Hello Again Angular!”`.
Mainly copyedits, but also
- Dart .jade extends TS .jade file with minor overrides
- Significant update of example code (so it matches the ts example in its
appearance and behavior).
- Tweaks to Dart code.
- A few extra/corrected mixin definitions in `_util-fns.jade`.