Add support for not including platform.js in build output

pkg/polymer/test/build/common.dart

Index: pkg/polymer/test/build/common.dart
diff --git a/pkg/polymer/test/build/common.dart b/pkg/polymer/test/build/common.dart
index c50efa64db18d370ac0bcffb82769eaa06420722..eed4c20069b4df310cb44b963dbfe5426a0dfc64 100644
--- a/pkg/polymer/test/build/common.dart
+++ b/pkg/polymer/test/build/common.dart
@@ -202,6 +202,8 @@ const EMPTY_DATA = '{"experimental_bootstrap":false,"script_ids":[]}';
 const WEB_COMPONENTS_TAG =
     '<script src="packages/web_components/platform.js"></script>\n'
     '<script src="packages/web_components/dart_support.js"></script>\n';
+const DART_SUPPORT_TAG =
+    '<script src="packages/web_components/dart_support.js"></script>\n';
 
 const INTEROP_TAG = '<script src="packages/browser/interop.js"></script>\n';
 const DART_JS_TAG = '<script src="packages/browser/dart.js"></script>';