Allow init() to work with Promises

[jahorton]

https://developers.google.com/web/fundamentals/getting-started/primers/promises

We want (and have) a non-blocking init, but there are certain API calls or page code setups that rely upon init() having been called and fail if we don't wait until initialization has already occurred. Promises provide a way to have Javascript reliably execute a block of explicitly post-initialization code and could be used to handle these cases.