Enqueue superclasses instead of supertypes.

tests/compiler/dart2js/array_static_intercept_test.dart

 // Copyright (c) 2011, 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';
 import 'package:async_helper/async_helper.dart';
 import 'compiler_helper.dart';
 
 const String TEST_ONE = r"""
 foo(a) {
   a.add(42);
   a.removeLast();
   return a.length;
 }
 """;
 
 main() {
   asyncTest(() => compile(TEST_ONE, entry: 'foo', check: (String generated) {
     Expect.isTrue(generated.contains(r'.add$1('));
     Expect.isTrue(generated.contains(r'.removeLast$0('));
-    Expect.isTrue(generated.contains(r'.length'));
+    Expect.isTrue(generated.contains(r'.length'),
+        "Unexpected code to contain '.length':\n$generated");
   }));
 }