The interesting thing is that Google has gone about the introduction of Dart in a scrupulously open way. They publicized the language early on it's design and implementation, they open sourced their implementation immediately on announcement and continued development in the open with input and contributions from all comers. The goal of the project is make it easier to develop better web apps. Now they've begun a standardization process.
So I expect Mozilla will oppose Dart, but they'll have to twist themselves into rhetorical contortions in the process. NIH is alive and well.