Remove MirrorsUsed and Comment from dart:mirrors before Dart 2.0 release

[rmacnak-google]

Dart2JS no longer implements mirrors; as such its cruft should be removed from mirrors.

• MirrorsUsed was added only for dart2js to inform its tree shaker.
• Comment was only used internally by dart2js, and it was never possible to get an instance at runtime.

[sigmundch]

For a transitional period, we still allow importing dart:mirrors from dart2js, but we fail at runtime. That means that using the @MirrorsUsed annotation is ignored. If we remove it before we make importing mirrors a compile time error, we risk introducing an unintended breaking change.

We should time these changes to happen at once.

[kevmoo]

At a minimum, we should mark MirrorsUsed as deprecated ASAP IMHO

[anders-sandholm]

Fixed (MirrosUsed marked as deprecated) in cdc520f

[rmacnak-google]

The issue is for removal, not deprecation.

[Sunbreak]

MirrorUsed is still there. We would keep it or not?

[lrhn]

We still want to get rid of it, but there were other higher-priority issues in the Dart 2.0 release, so it didn't make it there.

It doesn't do anything anymore, so we should be able to remove it at any time (preferably along with some other deprecated declarations, in one fell breaking change).