Add channel support to api.dartlang.org

This CL adds the following URL schemes:
  - http://api.dartlang.org/docs/channels/{be,dev,stable}/latest/
  - http://api.dartlang.org/docs/channels/{be,dev,stable}/424242/

Our channel builders are archiving the dartdocs to the following location:
gs://dartlang-api-docs/channels/{be,dev,stable}/REVISION/