StringMap really ought to always enforce a string
key, and also ought to have an index property.
This currently breaks too much code in Angular, so
should be fixed in a follow up Pull Request
separate from this.
But do it during the build process for cjs.
Right now we only need this when we transpile from ts
directly to es5. This is only the case in our
cis build, as for our browser build we only transpile
from ts to es6 via ts and then use traceur to do
the rest.