Ignore unused classes when constructing set for deferred loading.

tests/compiler/dart2js_extra/deferred/deferred_unused_classes_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.
 
+// Test that deferred loader analyzer doesn't trip over unused classes.
+
 import "package:expect/expect.dart";
 import 'dart:async';
 
+@lazy import 'deferred_class_library.dart';
+
+const lazy = const DeferredLibrary('deferred_class_library');
+
+class Base {}
+
+class DerivedNotUsed extends Base {}
+
 main() {
-  // Test that runZoned returns the result of executing the body.
-  var result = runZonedExperimental(() => 499,
-    onRunAsync: (f) {
-      Expect.fail("Unexpected invocation.");
-    });
-  Expect.equals(499, result);
 }