dart2js: fix dependency tracking for deferred loading when types are only used

tests/compiler/dart2js_extra/import_b_23853.dart

Index: tests/compiler/dart2js_extra/import_b_23853.dart
diff --git a/runtime/lib/async_patch.dart b/tests/compiler/dart2js_extra/import_b_23853.dart
similarity index 72%
copy from runtime/lib/async_patch.dart
copy to tests/compiler/dart2js_extra/import_b_23853.dart
index 682ee8b10a4ec2d6b8496a5f06496f428819a00e..cbbf0426b98ad7ab4a83f1a4b4737560f74d8139 100644
--- a/runtime/lib/async_patch.dart
+++ b/tests/compiler/dart2js_extra/import_b_23853.dart
@@ -2,5 +2,7 @@
 // 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 "dart:_internal";
+// helper for 23853_test.dart
+import 'import_a_23853.dart';
 
+createA() => new A();