Allow running from Dart code.

[isoos]

For the dartdocs.org site, it would be better if we could run dartdoc without executing a new process. The current bin/dartdoc.dart does a lot of heavy lifting, and also one-off environment setup, and copying that is not straightforward.

Is there a way to simply call a method to run dartdoc repeatedly from the Dart code?

[jcollins-g]

Not possible at the moment; dartdoc has too many things depending on command line arguments that are assumed to be global.

[jcollins-g]

#1676 fixes this. With that PR I think it's at least feasible to run dartdoc without fork and exec. Issues with doing so should be filed as separate bugs after that lands.