Serialize this-types in NativeBehavior separately.

pkg/compiler/lib/src/serialization/serialization.dart

 // Copyright (c) 2015, 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.
 
 library dart2js.serialization;
 
 import '../common.dart';
 import '../common/resolution.dart';
 import '../constants/expressions.dart';
 import '../dart_types.dart';
@@ skipping 674 matching lines @@
     if (element == null) {
       throw new ArgumentError('Serializer._getElementDataObject(null)');
     }
     element = element.declaration;
     DataObject dataObject = _elementMap[element];
     if (dataObject == null) {
       if (!shouldInclude(element)) {
         /// Helper used to check that external references are serialized by
         /// the right kind.
         bool verifyElement(var found, var expected) {
+          if (found == null) return false;
           found = found.declaration;
           if (found == expected) return true;
           if (found.isAbstractField && expected.isGetter) {
             return found.getter == expected;
           }
           if (found.isAbstractField && expected.isSetter) {
             return found.setter == expected;
           }
           return false;
         }
@@ skipping 424 matching lines @@
 
   /// Returns the value used to store [key] as a property in the encoding an
   /// [ObjectValue].
   ///
   /// Different encodings have different restrictions and capabilities as how
   /// to store a [Key] value. For instance: A JSON encoding needs to convert
   /// [Key] to a [String] to store it in a JSON object; a Dart encoding can
   /// choose to store a [Key] as an [int] or as the [Key] itself.
   getObjectPropertyValue(Key key);
 }