Fix a pretty bad bug of a class inheriting a patched class. The fix is

tests/language/issue10561_test.dart

+// Copyright (c) 2013, 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.
+
+// Regression test for dart2js that used to miscompile classes
+// extending HashMap, because HashMap is patched.
+
+import "package:expect/expect.dart";
+
+import 'dart:collection';
+
+class Foo extends HashMap {
+}
+
+main() {
+  Expect.equals(0, new Foo().length);
+}