exploring dart code completion

Not for review - exploring dart code completion. This is modeled after the code here: http://plnkr.co/edit/6MVntVmXYUbjR0DI82Cr?p=preview.

@rmsmith, to ping a old issue (https://github.com/rmsmith/ace.dart/issues/15), I think we can work around the proxy to a dart object issue, by creating a JS object with dart closures as properties. I'm happy to open a new issue against ace.dart for this - I think it'll require some changes to Ace's API in order to plumb it in.