Fix bug in dart2js spotted by the change:

tests/language/is_interfaces_test.dart

Index: tests/language/is_interfaces_test.dart
===================================================================
--- tests/language/is_interfaces_test.dart	(revision 0)
+++ tests/language/is_interfaces_test.dart	(revision 0)
@@ -0,0 +1,33 @@
+// Copyright (c) 2012, 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.
+
+class A {
+}
+
+class B extends A {
+}
+
+class C implements B {
+}
+
+int inscrutable(int x) => x == 0 ? 0 : x | inscrutable(x & (x - 1));
+
+main() {
+  var things = [new A(), new B(), new C()];
+
+  var a = things[inscrutable(0)];
+  Expect.isTrue(a is A);
+  Expect.isFalse(a is B);
+  Expect.isFalse(a is C);
+
+  var b = things[inscrutable(1)];
+  Expect.isTrue(b is A);
+  Expect.isTrue(b is B);
+  Expect.isFalse(b is C);
+
+  var c = things[inscrutable(2)];
+  Expect.isTrue(c is A);
+  Expect.isTrue(c is B);
+  Expect.isTrue(c is C);
+}