Reapply "Improve hashCode for closure objects" with fixes.

runtime/vm/dart_entry.h

Index: runtime/vm/dart_entry.h
diff --git a/runtime/vm/dart_entry.h b/runtime/vm/dart_entry.h
index d3aad80980fef470d2b0e6d8133b84e5680ee0bf..8728dd1b94898c0a2ece763a65b56e8b4d759651 100644
--- a/runtime/vm/dart_entry.h
+++ b/runtime/vm/dart_entry.h
@@ -183,6 +183,9 @@ class DartLibraryCalls : public AllStatic {
   // 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);