It's hard to discount the effect of momentum at this stage. In some respects Google is just tardy to the party. The ECMAScript vs whatever discussions happened circa 1995, the decisions have been made, and there's 20 years of near universal adoption there.
Then again...browsers will do what they need to do to keep users. If Dart allows devs to build things that aren't possible in js, then devs will lean on users to use browsers that are compatible with their newly built sites. Users flocking to a new browser will make the other browsers catch up.
So, it all starts with the capabilities of Dart itself. Make it kick javascript's butt, put it in chrome by default, and the rest should take care of itself.
Then again...browsers will do what they need to do to keep users. If Dart allows devs to build things that aren't possible in js, then devs will lean on users to use browsers that are compatible with their newly built sites. Users flocking to a new browser will make the other browsers catch up.
So, it all starts with the capabilities of Dart itself. Make it kick javascript's butt, put it in chrome by default, and the rest should take care of itself.