Of course people won't ship something that doesn't work cross-browser today. But by that logic, no proprietary feature is worth worrying about. Consider a hypothetical future in which Chrome had 90% market share: as a Web dev, would you bother shipping or testing the JS fallback?
If Chrome gets 90% marketshare, there'll be other problems besides Dart. History also tells us that even if all vendors adhere to standard specs, there will be implementation differences and bugs that still require developers to write browser specific code and fixups.
Again, I'm not disagreeing that there is a danger of fragmentation, but this won't happen because of Dart, but because of other forces. The criticism over the DartVM is being blown way out of proportion.
The best defense against any of this happening is not to protest Dart, but to make sure the browser market is competitive.
History tells us what will happen.