Devoncarew gen script

[devoncarew]

• create a tool script (doc_packages.dart) to generate docs for published packages. It can do one page of packages at a time (50 packages)
• change the stdout to print as we generate docs for a library, not as we create an individual html file. This can get too verbose for even moderate sized packages.
• fix two issues found when running the tool (NPEs)
• identified one more - will file an issue for it

@keertip @sethladd

[devoncarew]

fix #145

Will convert this to gen docs in parallel isolates in a future PR.

[devoncarew]

Can we turn off coveralls failing builds?

[keertip]

Looks good!

[sethladd]

Super cool. 🍨