chore(documentation-generation): prepare for transition from dartdoc to docgen

- Add preliminary support for `docgen`,
- But disable it for now due to a [problem with `docgen` when running on angular.dart sources](https://code.google.com/p/dart/issues/detail?id=16752).

This patch allows `dartdoc` to be used in stable builds and skips document generation in the dev build.

Fixes #552. Now both builds are green again.