You can get snapshots in JS just by structuring the code in a certain way (ala asm.js). You can get the same performance benefits by having a more limited subset (like Dart, no eval or dynamic code loading).

The real story is that Google wanted to go it alone rather than improve JS, and that's why there's Dart.

Actually dart2js compiles a "Dart-like" language to JavaScript. The "dart-like" doesn't include big integers or operator overloading on numbers.

They got some patches from the finance department and Google were really insistent on adding big integers to JS so it looks like they're using it for currency. Woo... exciting.