AFAIK TypeScript is a language which is a super-set of Javascript so in a sense it contains the flaws of it. Dart is a more ambitious project as it contains a completely new language, a VM, a development browser called Dartium including the VM for fast development cycle and debugging, a modularity system (pub), a quite large client and server side library, a stand alone editor and a javascript trnaslator (dart2js). It also works in server side.

Well, at the performance level, Dart VM is much much more faster than Javascript ones. I dont think Google plans to kill Javascript.

But Dart makes sense on Chromebooks for instance, where Google could deploy the Dart VM.

So the benefit is for Google ecosystem.

Front end devs are afraid of fragmentation , I think competition and choice is good. It will force JS to evolve much more faster, And why not ,maybe some day we will have a NodeDart too since Dart VM is open source, it would be awesome.