When I first got onto the Internet, nearly every page I came across had a little image at the bottom saying "Best Viewed in Netscape Navigator". Since I was using Netscape, everything worked fine and I didn't care. Some time later, I began to see images saying "Best viewed in Internet Explorer". Everything still worked fine, so I still didn't care... but as time went on, more of those images popped up, and the pages they appeared on began to look much, much scrappier in Netscape than they did in Explorer. While most of them still worked, more or less, Explorer made a noticeable improvement.
So, three cheers to Explorer for defeating Netscape and improving client-side development for everybody, right? Find a web-developer who still has to work with IE6 and ask them what they think.
Right now, Dart is freely-licensed and available to everybody, but who can contribute to it? Who will be allowed to veto compatibility-breaking changes? What happens if Apple announces some new hardware platform, and then Google coincidentally decides that supporting that platform in the Dart VM is no longer a priority, refuses to accept patches for it, and later refactorings happen to make it very difficult to maintain outside the official tree? What if the Dart VM becomes strategically important like Android, and current releases are only available to approved partners?
Sure, it's very unlikely that all those things would happen, and fairly unlikely that even one of them would: up until now, Google has stuck pretty close to the "do no evil" thing. But if we stick with openly, collaboratively created technologies, those things are impossible, and "impossible" is better than "unlikely" in my book.
If Dart were some kind of server-side thing Google wanted to use to increase the responsiveness of their sites, or something else internal to their systems, I wouldn't care — they're allowed to do whatever they want in the privacy of their own servers. It's only because the value proposition of Dart is "give Google an inherent advantage of control over the Web, and in exchange you will get shiny things" that I'm concerned. There'll always be more shiny things, but (so far) there's only one Web.
