Preliminary checked mirrors (not ready for review yet)

Preliminary checked mirrors (not ready for review yet)

This is by no means complete, and I haven't even used it with polymer yet, but you can check out some of the tests (like logging_test.dart) and examples to see the idea. All tests under test/from_sdk/ are just making sure the package passes all of the original dart:mirrors tests. The only changes there are to import this package instead.

The mechanism is simple, you import 'package:checked_mirrors/checked_mirrors.dart' instead of 'dart:mirrors', and we'll check things for you in Dartium. When deploying, we'll transform your code and replace the package-imports back with 'dart:imports', so this code goes away before dart2js compiles the app.