First try at removing ? from HTML.

tools/dom/templates/html/impl/impl_RTCPeerConnection.darttemplate

 // 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.
 
 part of $LIBRARYNAME;
 
 $(ANNOTATIONS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
 $if DART2JS
   factory $CLASSNAME(Map rtcIceServers, [Map mediaConstraints]) {
     var constructorName = JS('RtcPeerConnection', 'window[#]',
         '${Device.propertyPrefix}RTCPeerConnection');
-    if (?mediaConstraints) {
+    if (mediaConstraints != null) {
       return JS('RtcPeerConnection', 'new #(#,#)', constructorName,
           convertDartToNative_SerializedScriptValue(rtcIceServers),
           convertDartToNative_SerializedScriptValue(mediaConstraints));
     } else {
       return JS('RtcPeerConnection', 'new #(#)', constructorName,
           convertDartToNative_SerializedScriptValue(rtcIceServers));
     }
   }
 $endif
 
@@ skipping 28 matching lines @@
     var completer = new Completer<RtcSessionDescription>();
     _createAnswer(
         (value) { completer.complete(value); },
         (error) { completer.completeError(error); }, mediaConstraints);
     return completer.future;
   }
 $!MEMBERS
 }