I would like to use `transformPredefinedType`. Look at this https://github.com/vojtajina/traceur-compiler/commit/71cf4e967ddba6c2ea2859baac5bbb75cb495361 - I'm not sure how to get rid off this branching, which really makes it ugly.

Also, I realized that you could use `--types=true --annotations=true`, but without `--type-assertions=true`, in which case you don't expect `assert` library to be imported, but the type annotation is still generated and so we can't use `assert.string`. I think, we should do this:
1. use `transformPredefinedType` in `TypeTransformer`
2. transform `string` into `$traceurRuntime.string` (or `$traceurRuntime.types.string`)

What do you think?