Dart doesn't threaten the open web.
There isn't one browser dominating the market right now so web developers can't afford having their site work well in one browser only. Even if Chrome has an optimized Dart VM it wouldn't make sense for a website to depend on it given the current browser landscape.
I don't see Dart being adopted outright by other browser vendors but it may have an impact on future web standards. For instance if JavaScript ever gets class based OO perhaps Dart can serve as an inspiration for the standard.
It's okay for vendors to introduce non-standard features once in a while. If Netscape and Microsoft didn't try a bunch of crazy things in the early days the web wouldn’t be the same today. (think JavaScript and XMLHttpRequest)
On a side note I don't think Dart is a particularly cool language. But it's still better than translating Java to JavaScript as Google is doing with GWT.