Skip overriding members in API doc checks.

test/rules/public_member_api_docs.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // test w/ `dart test/util/solo_test.dart public_member_api_docs`
 
 abstract class A //LINT
 {
   static const Z = 1; //LINT
   static int _Z = 13; //OK
@@ skipping 32 matching lines @@
   F //OK
 }
 
 /// A D.
 abstract class D extends A {
   /// Make a D.
   D();
 
   @override
   a() => null; //OK
+
+  c() => null; // Un-annotated override -- OK!
 }
 
 foo() => null; //LINT
 
 /// Bar.
 bar() => null; //OK
 
 int g; //LINT
 
 int _h; //OK
 
 int gg, //LINT
    _gg; //OK