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!”`.
- Dart non-api doc pages w/ links to API entries will now have those
links translated from a TS API reference URL to a Dart API reference
URL.
- Created and ran a link checker to verify that all such links to API
entries refer to Dart.
- Fixed links to pages that have moved:
- JS & Dart: `i18n` is under now `cookbook` not `guide`
- Dart: testing page is now directly under guide.
Contributes to #1895Fixes#2273
