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
as much as I don't want to say this, this will break for SVG elements, because `className` is an `SVGAnimatedString`. As much as I don't want it to matter that it will break, people do use interpolation for SVG content attributes.