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

sdk/lib/html/html_common/conversions_dartium.dart

 part of html_common;
 
 convertDartToNative_PrepareForStructuredClone(value) =>
     new _StructuredCloneDartium().convertDartToNative_PrepareForStructuredClone(value);
 
 convertNativeToDart_AcceptStructuredClone(object, {mustCopy: false}) =>
     new _AcceptStructuredCloneDartium().convertNativeToDart_AcceptStructuredClone(object, mustCopy: mustCopy);
 
 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;
 }
 
 class _AcceptStructuredCloneDartium extends _AcceptStructuredClone {
   newDartList(length) => new List(length);
 
   // JsObjects won't be identical, but will be equal only if the underlying
   // Js entities are identical.
   bool identicalInJs(a, b) =>
       (a is js.JsObject) ? a == b : identical(a, b);
@@ skipping 31 matching lines @@
 final _promiseConstructor = js.context['Promise'];
 bool isJavaScriptPromise(value) => value is js.JsObject && value['constructor'] == _promiseConstructor;
 
 Future convertNativePromiseToDartFuture(js.JsObject promise) {
   var completer = new Completer();
   var newPromise = promise
     .callMethod("then", [(result) => completer.complete(result)])
     .callMethod("catch", [(result) => completer.completeError(result)]);
   return completer.future;
 }