This is nothing like IE, which is actually proprietary and closed-source. In contrast, there is absolutely nothing about Dart that's proprietary. Dart is 100% open source, including patents grants, it's completely developed in the open and for the last 2 years has seen steadily progress with feedback from the public mailing list. It's actually a goal for the project that everyone is able to freely use Dart in any capacity. Lars Bak already said that after 1.0 is shipped they're going to work on getting the language standardized. There is nothing legal or technical stopping other browser vendors using Dart, any hesitations are 100% political.
Dart is finally the first real alternative where we may be able to progress beyond JavaScript, that as a core objective also compiles to first-class JavaScript, i.e. many features dismissed because it couldn't be efficiently transpiled to JS. I care about progress, the current state of web development is fragmented and broken, we need to be encouraging more initiatives like Dart that challenges the status-quo and is able to progress past the fragmented inconsistent mess we have to deal with now in trying to develop complex, rich, high-performance web apps.