This half produces the JSON file by going through each of the HTML libraries
and pulling out the @DomName annotations as keys for the map.
This means that even though member names may change, the map still works.
Currently creates a blank json file because I have not hooked up the other half
that puts the docs into the libraries to begin with.
Review URL: https://codereview.chromium.org//12035057
git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@17610 260f80e4-7a28-3924-810f-c04153c831b5
rnystrom@google.com