Dart is faster because it lacks features that JavaScript has, such as dynamic loading or eval or adding fields to objects/classes, and has some special case hacks for SIMD.
One could easily tweak JavaScript to have the same lack of features and same performance gains (many could be done in asm.js style) so I do not see how Dart is relevant except as a way to not participate in the evolution of JavaScript.
Personally I feel that getting even a 50% boost on a scripting language that's never going to be as fast as C# or Java anyway, by making an additional incompatible VM, is totally foolish. Both VMs will be needed for many years, and this collateral bloat is just not worth the benefit. When you look at performance over JavaScript + asm.js the gains are seen largely where performance does not matter much anyway.
It makes a lot of sense if performance of Dart VM is about 1.5x or so faster. It is especially good for some of the math (or SIMD related things).
> You could remove all types from a TypeScript file and it'll probably run fine on any existing JS VM that supports ES6.
Right, so does Dart. So they are the same there.