First try at removing ? from HTML.

tools/dom/src/chrome/custom_dart/app_window.create.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // TODO(sashab): This override is no longer needed once prefixes are removed.
 void create(String url,
             [AppWindowCreateWindowOptions options,
             void callback(AppWindowAppWindow created_window)]) {
   void __proxy_callback(created_window) {
-    if (?callback)
+    if (callback != null)
       callback(new AppWindowAppWindow._proxy(created_window));
   }
   JS('void', '#.create(#, #, #)', this._jsObject, url, convertArgument(options),
      convertDartClosureToJS(__proxy_callback, 1));
 }