Google is clearly priming to do a Microsoft-style embrace-and-extend in a big way. Between Chrome, V8, NaCL, WebGL, Android, they have a wide range of technologies that they could begin steering away from standards.
But...why? Google makes money selling ads. It doesn't make money selling you support contracts for software that you've been locked into.
From what I can tell, Android exists so that Google isn't at the whim of Apple. The big G's greatest fear is that iPhones become the defacto smartphone, and Apple has the power to cut them out of their platform (by banning Google's ad networks and by setting the default search engine to Bing). I doubt the Android platform will ever really make money for Google.
When making these analyses people tend to forget that Google is a company of thousands of engineers. If we can devote a bit of engineering time to building better environments, the benefits across the entire company are multiplicative. Would you expect anything less from an engineering-driven company?
Don't forget SPDY. If you use Chrome, you're probably using SPDY when accessing services like GMail and Google Search. What does this get you? A snappier experience. This could dissuade you from using other services even in just the 'feel'.
Google wants to lock you into their advertising channels (gmail, google docs, google search).
As long as your mail app is a source of revenue for someone (whether from support contracts or ads), they benefit from you being locked in, at least locally. Globally they may suffer because of the loss of goodwill, of course.
Except that they have a monopoly position in none of those domains. Microsoft used their power to protect their monopoly. The Dart document itself says the number one reason for Dart is to protect Google's massive bet on the web platform. It's easy to see how Microsoft or Apple could seriously undermine Google by keeping the web platform second rate, which is certainly within their power given current browser marketshares.