Implement non-function type alias support in dart2js

[leafpetersen]

This issue tracks support of the non-function type alias feature. See the enclosing meta-issue for details.

I don't expect much work here, but we should validate that debugging/error messages work as desired, and that the backend doesn't choke.

cc @sigmundch @eernstg @mit-mit @franklinyow

[leafpetersen]

@sigmundch is there any work that we know of for this, or should I close it out?

[sigmundch]

I think we can close this out, I'm not aware of any work needed.

/cc @fishythefish - in case I missed something.