Support union arguments in new IDL

[vsmenon]

We're seeing syntax like the following in Blink IDL:

  [ImplementedAs=namedGetter] getter (NodeList or Element) namedItem(DOMString name);

This requires C++ code that handles both types to be generated.

I'm modifying the template to throw for now when this case is encountered at runtime.

[vsmenon]

Marked this as blocking #18364.

[kasperl]

Removed this from the 1.4 milestone.
Added this to the 1.5 milestone.

[vsmenon]

Terry, is there anything further to do here?

[vsmenon]

Unmarked this as blocking #18364.

[terrylucas]

named item getters are not fully support in the dart:blink code however, dart:html never uses blink for these so it's not critical for 1.5. We should fix for completeness after the 36 roll.

---

Removed Priority-Critical label.
Added Priority-Low label.

[vsmenon]

Removed this from the 1.5 milestone.
Added this to the 1.6 milestone.

[kasperl]

Removed this from the 1.6 milestone.
Added Oldschool-Milestone-1.6 label.

[kasperl]

Removed Oldschool-Milestone-1.6 label.

[terrylucas]

Closed unions are generated as dynamic until Dart supports either union or function overloading.