Comparing JS interop in Dart and TypeScript is apples and oranges. Dart has a native virtual machine with its own garbage collector and object representation. The language itself has an entirely different object model and set of collection types. For better or worse, even Dart's number types are different from JS. Doing JS interop in Dart is like trying to do interop between the JVM and CLR.

TypeScript doesn't have JS interop: it is JavaScript. TypeScript is basically JS linter with a type annotation syntax. (And a few additional local features like arrow functions and class syntax.)

That being said, we know on the Dart team that JS interop is hugely important. It's just much harder for us to do. We've just announced a big step in the right direction: http://www.dartlang.org/articles/js-dart-interop/