And now that Google is joining the fray we're concerned? Not at all. There will always be applications that only run on one browser (or operating system like Android). This same methodology will be incorporated by Google for gmail or any of their other services which will allow them to control which environment gets the best features and allow them to leverage their technologies effectively when their competitors are already doing the same thing.
Again, for this thread, per the leaked memo, that was not the plan with Dart. Coming to a standards body late will not work -- it may get a spec, but not Dart support in other browsers -- unless Googe has near-monopoly power.