Refactor Testability

[matanlurey]

Testability (i.e. the interface for integration testing) is sort of a mess.

We should take the following steps:

• [x] Write documentation on how it currently works (and what JS interfaces are present)
• [ ] Refactor to use package:js, instead of lots of hand-written JS interop
• [ ] Add some decent functional or integration testing, ourselves
• [ ] Write user-facing documentation on using it
• [ ] Make it optional (i.e. tree-shakable for production applications)

[matanlurey]

Docs landed in https://github.com/dart-lang/angular/pull/1189.