Now that Apple has successfully marginalized Flash as a cross platform development environment the browser fragmentation will ramp up and we'll see more of this type of behaviour rather than less. Apple didn't start this fragmentation but they are committed to ensuring that the web is not a level playing field with technology segmentation like canvas and css ES. Microsoft has always tried to fragment the market with IE. Take for example the current version of web based Outlook which only functions at full power in the IE browser, otherwise you get the 'lite' incarnation.
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.
You're not concerned about fragmentation so long as there's a "lite" fallback. Got it. Let's see how this plays out. It may be that browser vendors with hot websites have to make the "lite" experience as good over time as their proprietary one, so they work fairly with the standards bodies.
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.