Fix bug 5511:

runtime/lib/object_patch.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.
 
 patch class Object {
 
   // Helpers used to implement hashCode. If a hashCode is used we remember it
   // using an Expando object. A new hashCode value is calculated using a Random
   // number generator.
   static Expando _hashCodeExp = new Expando("Object.hashCode");
   static Random _hashCodeRnd = new Random();
 
   /* patch */ int get hashCode {
+    if (this == null) {
+      return 2011;  // The year Dart was announced and a prime.
+    }
     var result = _hashCodeExp[this];
     if (result == null) {
       result = _hashCodeRnd.nextInt(0x40000000);  // Stay in Smi range.
       _hashCodeExp[this] = result;
     }
     return result;
   }
 
   /* patch */ String toString() native "Object_toString";
   // A statically dispatched version of Object.toString.
@@ skipping 15 matching lines @@
   // Call this function instead of inlining instanceof, thus collecting
   // type feedback and reducing code size of unoptimized code.
   _instanceOf(instantiator,
               instantiator_type_arguments,
               type,
               bool negate)
       native "Object_instanceOf";
 
   /* patch */ Type get runtimeType native "Object_runtimeType";
 }