Added Library-Mirrors label.

This comment was originally written by @Andersmholmgren

For me the name is confusing because you can also pass a symbol for a method and it returns a closure mirror, which is what i wanted but getField is not a great name in that case. Maybe getMember or something would be better.

My test program

import 'dart:mirrors';

class Foo {
  final int blah;
  Foo(this.blah);

  void fum() {}
}

main() {
  final foo = new Foo(6);
  final fm = reflect(foo);
  final fumm = fm.getField(#fum);
  final ffm = reflect(foo.fum);
}

Given we don't plan to add a proper getField and setField (#7867), we won't do the breaking renaming the existing getField and setField.