feat(testability): add $testability service

The $testability service is a collection of methods for use when debugging
or by automated testing tools. It is available globally through the function
`angular.getTestability`.
For reference, see the Angular.Dart version at
https://github.com/angular/angular.dart/pull/1191
No, you don't want to look at the root element.

There are alternatives: `matches()` (which might be `msMatches()`, `mozMatches()`, or `webkitMatches()`, etc), or just `jqLiteHasClass` for the parent element