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
Hm, this will only be an issue if the user has set their root element to an svg element (seems very unlikely). In fact, I wonder if including the root element in the binding search is reasonable at all. I added this because without it one test fails, but that test was an odd case, where there was a `{{stuff}}` directly inside the `<body>`.