[Enum Shorthands] Dart2JS Implementation

[kallentu]

This issue tracks work needed to implement enum shorthands in Dart2JS.

From the CFE prototype I've been working on, I'll most likely be rewriting enum shorthand nodes into the full typed expressions (so .blue -> Color.blue). The backends won't really know the difference, having given the full expanded reference, and shouldn't need work on their end.

I'll keep the issue open in case we think of anything else or if anything changes.

cc. @sigmundch @natebiggs