dart2js: consider letting MirrorsUsed metatarget "hit" things annotated with the type

[kasperl]

Right now, if you use a metatarget for MirrorsUsed:

   @­MirrorsUsed(metaTarget: 'X')
   import 'dart:mirrors';

it doesn't apply to things annotated with the X type, but only things annotated with an instance of X. So in this case:

   @­X
   foo() { ... }

   @­X()
   bar() { ... }

we only consider the bar function as being "hit" by the metatarget specification. Maybe we should reconsider that.

[DartBot]

This comment was originally written by @seaneagan

---

looks like another symptom of issue #13582

[DartBot]

This comment was originally written by @seaneagan

---

Status:Invalid given issue #14647?

[kasperl]

Yeah, this is invalid (for now). Thanks for noticing, Sean!

---

Added Invalid label.