Moved ChromiumSubscribeUniform to web_gl

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"]);
@@ skipping 102 matching lines @@
     }
   }
 
   Map get toMap => _values;
 
   _ReturnedDictionary(Map value): _values = value != null ? value : {};
 }
 
 // Helper function to wrapped a returned dictionary from blink to a Dart looking
 // class.
-convertNativeDictionaryToDartDictionary(Map values) => new _ReturnedDictionary(values);
+convertNativeDictionaryToDartDictionary(Map values) =>
+  values != null ? new _ReturnedDictionary(values) : null;
 
 // Conversion function place holder (currently not used in dart2js or dartium).
 List convertDartToNative_StringArray(List<String> input) => input;
 
 // Converts a Dart list into a JsArray. For the Dartium version only.
 convertDartToNative_List(List input) => new js.JsArray()..addAll(input);
 
 /// Find the underlying JS object for a dart:html Dart object.
 unwrap_jso(dartClass_instance) => js.unwrap_jso(dartClass_instance);
 
@@ skipping 268 matching lines @@
       {'type': dartClass, 'extends': extendTag != null ? extendTag : "" };
 }
 
 Type getCustomElementType(object) {
   var entry = getCustomElementEntry(object);
   if (entry != null) {
     return entry['type'];
   }
   return null;
 }