Dart is better in most respects. The standard library is light years ahead of JS's, the Dart tooling is fantastic, it has sound typing.

There are a couple of features that I miss from Typescript: the biggest by far is tagged unions.

But I would still pick Typescript for a web project for two reasons:

* The community is like 1000x bigger. Even though a lot of it produces extremely low quality code, there's still a lot more solid JS libraries than Dart ones.

* Debugging is slightly easier since the compiled JS code is essentially identical to you TS code. Chrome has pretty great support for Dart, but if you ever have to delve into the generated JS it is quite painful.