The intercepted classes set contains interceptor classes, not the classes they intercept.

tests/language/interceptor7_test.dart

Index: tests/language/interceptor7_test.dart
===================================================================
--- tests/language/interceptor7_test.dart	(revision 0)
+++ tests/language/interceptor7_test.dart	(revision 0)
@@ -0,0 +1,14 @@
+// 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.
+
+// Test that dart2js uses the right interceptor when call a method on
+// something that has type number.
+
+var array = [];
+
+main() {
+  array.add(false);
+  var x = array[0] ? 1.5 : 2;
+  Expect.isTrue(x.isEven);
+}