[js-interop] Update js.dart to new trunk (r17072)

- Used mappedBy instead of map (as in trunk)
- Use browser package

Note: testing is going to be painful until the next editor integration roll and unittest pub upload.  To test this, I'm using the trunk editor build (http://gsdview.appspot.com/dart-editor-archive-trunk/latest/) and changing the packages/unittest symlink by hand to point to the version in the editor's dart-sdk/pkg directory.  I won't upload a new version to pub until the next integration roll.

Review URL: https://chromiumcodereview.appspot.com//11858024