Feature Request: Don't fire callback until load callback from google maps fires

In our project, we've seen a bunch of "google maps is undefined errors" on mobile devices that seem to have slow connections, and my current hypothesis is the following:

1. Google maps script is injected
1. Bootstrap code is downloaded (i.e. not the library code itself)
1. script onLoad event fires before library code has finished loading
1. Google Maps is treated as loaded, but `window.google.maps` is still null/undefined.

One fix for the above behavior would be to take advantage of the [callback parameter](https://developers.google.com/maps/documentation/javascript/tutorial#Loading_the_Maps_API) on the google maps JS request.

Is this consistent with your experience? Are there challenges to implementing this? Otherwise I'd be happy to submit a PR.

No file or test is named in the issue. Start by locating the Google Maps script injection and its script onLoad handling, then trace how the loader invokes its callback. Done means the callback waits for the documented Google Maps loading callback rather than assuming window.google.maps is ready.

Written by the indexing model from the issue text.