Generate docs for the core SDK

[sethladd]

Building docs for the core SDK might be different from generating docs for a random package. We should ensure that this tool works for internal and external docs.

[sethladd]

Should we close this?

[keertip]

fixed in #110

[alan-knight]

What does this do for e.g. dart:html. I was noticing that it's suppressed for analyzer tests because it gives errors. Or are the errors not enough to prevent analysis for doc generation purposes?