Polyfill only once

pkg/dev_compiler/tool/input_sdk/private/js_helper.dart

Index: pkg/dev_compiler/tool/input_sdk/private/js_helper.dart
diff --git a/pkg/dev_compiler/tool/input_sdk/private/js_helper.dart b/pkg/dev_compiler/tool/input_sdk/private/js_helper.dart
index eb932e36bec893ff25712425f124fbef708da2f5..cbc52c6f0479931c69aa56f5fe212a002f7bda3a 100644
--- a/pkg/dev_compiler/tool/input_sdk/private/js_helper.dart
+++ b/pkg/dev_compiler/tool/input_sdk/private/js_helper.dart
@@ -910,6 +910,7 @@ class BooleanConversionAssertionError extends AssertionError {
 // Hook to register new global object.  This is invoked from dart:html
 // whenever a new window is accessed for the first time.
 void registerGlobalObject(object) {
-  dart.polyfill(object);
-  dart.applyAllExtensions(object);
+  if (dart.polyfill(object)) {
+    dart.applyAllExtensions(object);
+  }
 }