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
good catch. Both work on url now (everything past the authority, e.g. /path?a=b#hash)