add reflectable to object used in html page

The push dialog doesn't work when compiled to javascript. From the exceptions, the `CommitInfo` object doesn't have any of the fields that the polymer UI is trying to access on it (`message`, `identifier`, ...). I think dart2js is mangling the field names.

As a policy, we should always add `@observable` or `@reflectable` to anything accessed via html.

@dinhviethoa, @ussuri