Some random cleanup

Review URL: https://github.com/google/traceur-compiler/pull/481

Closes #481.
Refactor ModuleTransformer

This is to make it easier to subclass and override what code to generate.

See issue #461 for motivation.

Review URL: https://github.com/google/traceur-compiler/pull/482

Closes #482.
Merge remote-tracking branch 'upstream/master' into types

* upstream/master:
  Refactor ModuleTransformer
  Some random cleanup