dart2js: Make sure that super classes have their inheritance chain set up correctly.

tests/language/inheritance_chain_lib.dart

+// Copyright (c) 2015, 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 "inheritance_chain_test.dart";
+
+class B extends C {
+  get id => "B";
+  get length => 2;
+}
+
+class D extends Z {
+  get id => "D";
+  get length => 4;
+}
+
+class W {
+  get id => "W";
+  get length => -4;
+}
+
+class Y extends X {
+  get id => "Y";
+  get length => -2;
+}