This can be used to retain any properties that you wish to access with
Dart's mirror system. If you import package:observe/mirrors_used.dart, all
classes or members annotated with @reflectable wil be preserved by dart2js
during compilation. This is necessary to make the member visible to
PathObserver, or similar systems, once the code is deployed, if you are
not doing a different kind of code-generation for your app. If you are using
polymer, you most likely don't need to use this annotation anymore.