Make generators not depend on runtime.js

Use RuntimeInliner to generate addIterator and getIterator instead.

To make this work we use a string key for the iterator symbol if
traceur.options.privateNames is false (private names require
runtime.js anyway).

Review URL: https://github.com/google/traceur-compiler/pull/412

Closes #412.
Remove support for optional commas in object literals

Fixes #404

Review URL: https://github.com/google/traceur-compiler/pull/422

Closes #422, closes #404.
Remove some old unused predefined strings.