Adding MirrorsUsed attribute to dart:mirrors import

It's only using symbols, so just enabling that. For the unittests, this takes the size from ~4.5MB down to ~670kB.
Merge pull request #131 from blois/mirrors_used

Adding MirrorsUsed attribute to dart:mirrors import
clarify the code size implications of package:js
bump pubspec.yaml
change versions for dart-1.0