Generate needed classes for constant maps in metadata.

tests/lib/mirrors/metadata_const_map_test.dart

 // Copyright (c) 2014, 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 that classes referenced from a signature of a tear-off closure
-// are emitted.
+// Regression test for issue 20776. Tests that the needed classes for the
+// constant map in the metadata are generated.
 
-@MirrorsUsed(targets: 'C')
+library lib;
+
+@MirrorsUsed(targets: 'lib')
 import 'dart:mirrors';
 
-import 'package:expect/expect.dart';
+class C {
+  final x;
+  const C(this.x);
+}
 
+@C(const {'foo': 'bar' })
 class A {}
 
-class C {
-  A foo() {}
-}
 
 main() {
-  Expect.isFalse(reflect(new C().foo).function.returnType.toString()
-      .contains('dynamic'));
-}
+  print(reflectClass(A).metadata);
+}