Reapply "Improve hashCode for closure objects" with fixes.

runtime/vm/dart_entry.h

 // Copyright (c) 2011, 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.
 
 #ifndef RUNTIME_VM_DART_ENTRY_H_
 #define RUNTIME_VM_DART_ENTRY_H_
 
 #include "vm/allocation.h"
 #include "vm/growable_array.h"
 
@@ skipping 165 matching lines @@
 
   // On success, returns a RawInstance.  On failure, a RawError.
   static RawObject* ToString(const Instance& receiver);
 
   // On success, returns a RawInstance.  On failure, a RawError.
   static RawObject* HashCode(const Instance& receiver);
 
   // On success, returns a RawInstance.  On failure, a RawError.
   static RawObject* Equals(const Instance& left, const Instance& right);
 
+  // On success, returns a RawInstance.  On failure, a RawError.
+  static RawObject* IdentityHashCode(const Instance& object);
+
   // Returns the handler if one has been registered for this port id.
   static RawObject* LookupHandler(Dart_Port port_id);
 
   // Returns null on success, a RawError on failure.
   static RawObject* HandleMessage(const Object& handler,
                                   const Instance& dart_message);
 
   // Returns null on success, a RawError on failure.
   static RawObject* DrainMicrotaskQueue();
 
   // map[key] = value;
   //
   // Returns null on success, a RawError on failure.
   static RawObject* MapSetAt(const Instance& map,
                              const Instance& key,
                              const Instance& value);
 };
 
 }  // namespace dart
 
 #endif  // RUNTIME_VM_DART_ENTRY_H_