Annotations should always invoke the constructor

[floitschG]

@jacob314 suggests that annotations should always invoke the constructor:
From #26322 (comment):

This is a general issue with annotations in Dart2JS and Dart. I think the most user friendly solution is to have
@AnyAnnotation
be syntactic sugar for
@AnyAnnotation()
if AnyAnnotation is a class.
This is consistent with how some other languages handle annotations, makes the code users write more terse, and avoids these entirely confusing compile errors altogether.

[munificent]

Duplicate of #13582.