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
Well, `querySelectorAll` only finds child elements - the idea here was to take a look at the root element itself. But I'm convincing myself that's a bad idea. I'm taking a look at just removing it.