[Dart] rename Angular 2 to simply Angular

[chalin]

• TS prose and sample code and README etc chore: rename Angular 2 to simply Angular #2402, docs(ts-to-js): change angular2 to @angular #2403
• _data.json files docs: fix {ts,dart,js}/latest/_data.json #2387
• _cache - chore(dart): complete the renaming of "Angular 2" to "Angular" #2820
• Dart prose - chore(dart): complete the renaming of "Angular 2" to "Angular" #2820
• Dart sample - chore(dart): complete the renaming of "Angular 2" to "Angular" #2820
  • Beware that we cannot just sweep the code and replace angular2 with angular because the plan is to keep the angular2 library name in Dart.
• wiki pages?

[chalin]

cc @kwalrath