Inline annotations for optional parameters requires ES6 syntax

I think this should be supported simply by this:

`function foo(/** (string|undefined) */ optional) { ... }`

since calling `foo()` with no arguments will give the `optional` parameter the `undefined` value.

On the other hand, maybe this could be supported as well:

`function foo(/** string= */ optional) { ... }`

Currently this generates no warning or error, but the `=` seems to be silently ignored.