I hope Google will provide some iron clad guarantees that their Dart project will be around for the next 5 years
5 years is a lifetime in web technology. 5 years ago nobody was using coffeescript, angular, ember, or any of that. They didn't exist, or if they did they were crude ancestors of what they are today.
I just don't think you can have this expectation of any guarantees like that in web development. Things move too fast. If you want that kind of guarantee, use open source stuff and acknowledge that you'll have to support it yourself when the rest of the world moves on.
Here's my problem with this line of thinking: Dart is supposed to replace one third of the triad of core technologies web developers use: HTML, JS, and CSS. It doesn't appear that in the long term it's intended to be transcompiled to JavaScript. CoffeeScript is only intended to be an alternative that transcompiles to JS. There's no effort to build a CoffeeScript VM. (C8? CoffeeMonkey?)
Angular, Ember, etc are just frameworks. They're not likely to be as long-lived as the triad of front-end web development technologies because the creation of such a framework has a significantly lower barrier to entry and adoption. There's no W3C committee overseeing development and there will be no W3C draft or standard for any of these somewhat ephemeral frameworks.
Core web development technologies are very stable and their development, deployment, and adoption happens quite slowly; on the order of decades. If Dart does become widely adopted (and with the support of browser markers) it'll likely remain in place for a long, long time, far longer than the latest JavaScript framework or veneer language de jure.
Even if Dart does become widely adopted, Dart hasn't got a hope of truly replacing JavaScript, at least for the foreseeable future (>10 years), as both a Dart VM and a JS VM would have to coexist in all browsers for reasons of backward compatibility. Yay, browser codebase bloat!
5 years is a lifetime in web technology. 5 years ago nobody was using coffeescript, angular, ember, or any of that. They didn't exist, or if they did they were crude ancestors of what they are today.
I just don't think you can have this expectation of any guarantees like that in web development. Things move too fast. If you want that kind of guarantee, use open source stuff and acknowledge that you'll have to support it yourself when the rest of the world moves on.