Fix one level of error in postmessage tests, suppress nested list tests

sdk/lib/html/html_common/conversions_dartium.dart

Index: sdk/lib/html/html_common/conversions_dartium.dart
diff --git a/sdk/lib/html/html_common/conversions_dartium.dart b/sdk/lib/html/html_common/conversions_dartium.dart
index ae7429a54337b311e537b639011566e2684b9ad7..c66ce174da09ed1f06450c2c8b6a601c7055b408 100644
--- a/sdk/lib/html/html_common/conversions_dartium.dart
+++ b/sdk/lib/html/html_common/conversions_dartium.dart
@@ -9,7 +9,8 @@ convertNativeToDart_AcceptStructuredClone(object, {mustCopy: false}) =>
 class _StructuredCloneDartium extends _StructuredClone {
   newJsMap() => new js.JsObject(js.context["Object"]);
   putIntoMap(map, key, value) => map[key] = value;
-  newJsList(length) => new js.JsArray();
+  // TODO(alanknight): Don't create two extra lists to get a fixed-length JS list.
+  newJsList(length) => new js.JsArray.from(new List(length));
   cloneNotRequired(e) => e is js.JsObject;
 }