support a single dartdoc-viewer instance for all packages

[sigmundch]

Not sure if this is feasible, but could we reuse the same app instance for all packages? That would mean invoking docgen without the --compile flag, and separately building the app only when you'd like to update it.

There are some tradeoffs of course: the good thing is that breakages like #80 would be less likely to happen (you'd push a new version of the app and validate that it works before doing so), you also get to save some space as you only need to upload new .json files for each package.

The bad news is that the process of updating the viewer is more manual, and changes in the .json format would have to be coordinated more carefully.

Thoughts?

[adam-singer]

@sigmundch I don't think it would save any space since all packages have their own dartdoc-viewer. Each time a build happens all resources throw away for the most part. I wish the --compile flag was more flexible/stable than always picking the head of master on github. Maybe --compile should support versions or github tags, since it triggers a git clone for the dartdoc-viewer.

I'll consider doing a single dartdoc-viewer. It would solve breakages from the client viewer builds but it wouldn't solve breakages if docgen decides to change or introduce new format. The dartdocs nodes always pull http://storage.googleapis.com/dart-archive/channels/dev/release/latest/sdk/dartsdk-linux-x64-release.zip. So thats also something to consider.

I was hoping that dartdocs could be minimal maintenance by following latest release which should be latest of docgen and dartdoc-viewer.