Load all Maps API libraries by default (proposal)

[brendankenny]

Due to the way the Maps API loads libraries (designed in the ancient days of ~2008) you have to load all the libraries you'll need on first load and can't load more libraries later. You also need to make sure the API is only loaded once on each page.

The second problem is taken care of by <google-maps-api> but the first means that every other element that uses it needs to specify the same set of libraries as the rest for them to work on the same page together. This can be problematic if you're trying to make a SPA and have a need for different libraries in different parts of your app, or if you don't even realize an element you're using contains a <google-maps-api> and so needs a libraries property properly set. We leave it up to the end developer to coordinate this (see e.g. https://github.com/GoogleWebComponents/google-streetview-pano/issues/15). Once you know the problem exists, it's easy to diagnose and fix, but it's annoying.

To eliminate these problems, I propose we always load all four libraries whenever the Maps API is loaded and eliminate the libraries property on all google-map-* elements. Configuration gets simpler, you don't have to repeat yourself over and over to use the exact same libraries you already loaded, and we eliminate a class of bugs. Boom.

Downsides:

• libraries are loaded as separate JS files, requiring four more network requests dependent on the bootstrap script loading and executing
• some amount of parse and initialization time for the libraries, even if unused (beyond what the JS engine has to do because of the way the Google Maps loader works)

Why the downsides aren't so bad:

• total download size of the four libraries is ~5.5KB with gzip (~9.5KB after unzip)
• there are already 15 scripts loaded with a vanilla Maps API load that we weren't worrying about before :)
• the library script downloads are launched immediately by the bootstrap script, in parallel to the main.js script file
• Maps API files are served with QUIC or HTTP2 or SPDY where available, so extra requests are multiplexed over an already open connection

Any possible network performance problem should be fixed at the browser/Maps API/serving infrastructure levels anyways. We shouldn't burden all the google-map-* components with this additional API complexity.

[brendankenny]

(this should maybe be merged into #22, but the suggestion there will not work because of the temporal aspect of the problem: the only union of libraries that will include all possible libraries needed by elements that could be added to the page in the future is the set of all libraries :)

[ebidel]

Want to close and dump https://github.com/GoogleWebComponents/google-apis/issues/57#issue-109014729 into #22?