Some people wanted to build optional-typing language for real world for decades, and finally made it. That is a great achievement.

From product point of view, Google would be much better to port Android Dalvik to Chrome. You can run GWT Java code in browser years ago. Running the same code using JVM would be much faster with much less code change. 5-10 man-year would be enough to do the porting, and it will share work between Android and Chrome for years to come. Of course, it would be much boring research work though.

From performance point of view, asm.js gets much better performance with way less work. No matter how good Dart performs, it would never make sense to port high performance C/C++ game code to Dart, so you end up having to use both Dart + pNaCl, which is an awful solution. Technically, TypeScript is more interesting if Microsoft can keep it simple and fast, but I doubt Microsoft would make anything simple.