"We have a language that works only/best in our browser, features that work only/best in our browser, a dominant OS which requires automatically includes/installs our browser."
I guess it was a good strategy for Microsoft while it lasted (and still pays benefits) so they might as well copy it. It will be funny if the EU requires Android phones / tablets to give you a 'browser choice' when you first turn them on / install them.
To be very clear, Dart runs across the modern web when you compile it to JavaScript, so it's not a language that works only in one browser.
What stops sites from shipping Dart only and not shipping the JavaScript?
That's right, and historically encouraging authors to use vendor-specific stuff by making that the easiest thing to do, whether it be filter:progid:DXImageTransform, WebKit prefixes, ActiveX banking plugins, or what have you, has been bad for user choice.
To give just one example, WebKit asked authors not to rely on WebKit prefixes, but they did and now we're in the situation where just yesterday I wasn't able to use clippercard.com on Firefox for Android because the design was so broken as to be unusable. :(
I agree that sites should do it, but developers have not done this in the past. If Chrome has a commanding market share, history says that developers will not bother to target other browsers.