Re-apply https://codereview.chromium.org/14636002/, but this time avoid creating TypeMask with dyna…

tests/language/abstract_object_method_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.
+
+import "package:expect/expect.dart";
+
+class A {
+  noSuchMethod(_) {
+    Expect.fail('Should not reach here');
+  }
+}
+
+class B extends A {
+  operator ==(other);
+}
+
+class C extends B {
+}
+
+var a = [new C()];
+
+main() {
+  C c = a[0];
+  a.add(c);
+  Expect.isTrue(c == a[1]);
+}