Make RTC work on Firefox (mostly).

tools/dom/templates/html/dart2js/impl_RTCSessionDescription.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 {
   factory $CLASSNAME(Map dictionary) {
-    return JS('RtcSessionDescription', 'new RTCSessionDescription(#)',
+    var constructorName = JS('RtcSessionDescription', 'window[#]',
+        Device.isFirefox ? '${Device.propertyPrefix}RTCSessionDescription' : 
+        'RTCSessionDescription');
+    return JS('RtcSessionDescription', 
+        'new #(#)', constructorName,
         convertDartToNative_SerializedScriptValue(dictionary));
   }
 $!MEMBERS
 }