union types should be added to commentary?

[sigmundch]

from: dart-archive/core-elements#9

Until this bug is fixed: https://code.google.com/p/dart/issues/detail?id=4938
we can't represent union types in Dart, so we'd need to include them in a doc comment if we want to keep that level of precision.

personally, I'm not sure this is worth it. I'd rather redesign APIs to avoid union types where it makes sense.