Do not use webkit prefix for postMessage.

tests/html/transferables_test.dart

Index: tests/html/transferables_test.dart
diff --git a/tests/html/transferables_test.dart b/tests/html/transferables_test.dart
index ff1b9be4adba1708a13fb93f0c67e2a8686d045f..220852cc25ef0a01d6bd901fc7060ad27e684156 100644
--- a/tests/html/transferables_test.dart
+++ b/tests/html/transferables_test.dart
@@ -15,6 +15,6 @@ main() {
       expect(messageEvent.data, new isInstanceOf<ArrayBuffer>());
     }));
     final buffer = (new Float32Array(3)).buffer;
-    window.webkitPostMessage(buffer, '*', [buffer]);
+    window.postMessage(buffer, '*', [buffer]);
   });
 }