Return Promise from Loader.import/module/define/loadAsScript/script.

API changes: no callback args.

To update code, replace any callback arguments with .then(callback, errback):
  loader.import(name, {}, callback, errback);
become
  loader.import(name, {}).then(callback, errback);

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/747

Closes #747.