add support for the new super mixins

[devoncarew]

This has two parts:

• updating the backend with analysis and compile support, when the implementations are ready
• updating the front-end with the new syntax highlighting; this depends on a new publish of codemirror.dart, which itself needs a new release of the codemirror library (the change to codemirror JS is already in, and releases can be tracked here: https://github.com/codemirror/CodeMirror/releases)

[devoncarew]

Overall feature tracking issue here: dart-lang/language#12

[jcollins-g]

mixin keyword will not be highlighted until this gets done.

[jcollins-g]

Backend is working, and even without this the supermixin support is probably enough for day one, so not a blocking bug.

[devoncarew]

done?

[jcollins-g]

@devoncarew I thought there was some syntax highlighting that we wanted, even if it wasn't completely blocking

[jcollins-g]

if the syntax highlighting is a blocker let me know and I can try and chase it down.

[devoncarew]

Hmm, yes, I forgot. We need to update (https://github.com/google/codemirror.dart/blob/master/updating_codemirror.md) codemirror.dart to the latest, and then publish that package.

let me know and I can try and chase it down

If you'd like to do it, sounds great; I'm having some issues running (and updating) bower on my machine

[RedBrogdon]

CodeMirror was updated as a side effect of our work on the new embed UI. It looks like this is working to me.

Just tested with:

class blah with Musical {}

mixin Musical {
  bool canPlayPiano = false;
  bool canCompose = false;
  bool canConduct = false;

  void entertainMe() {
    if (canPlayPiano) {
      print('Playing piano');
    } else if (canConduct) {
      print('Waving hands');
    } else {
      print('Humming to self');
    }
  }
}

void main() {
  final b = blah();
  b.entertainMe();
}

I'm seeing correct highlighting for the mixin keyword, and no weird warnings/errors.