Fix braino in SSA optimizer and add a test for it.

sdk/lib/_internal/compiler/implementation/lib/interceptors.dart

 // Copyright (c) 2012, 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 _interceptors;
 
 import 'dart:collection';
 import 'dart:collection-dev';
 import 'dart:_js_helper' show allMatchesInStringUnchecked,
                               Null,
@@ skipping 68 matching lines @@
 /**
  * The interceptor class for [Null].
  */
 class JSNull implements Null {
   const JSNull();
 
   // Note: if you change this, also change the function [S].
   String toString() => 'null';
 
   int get hashCode => 0;
-  Type get runtimeType => createRuntimeType('Null');
+  Type get runtimeType => Null;
 }