Add Dart_IdentityHash to the embedding API.

runtime/include/dart_api.h

 /*
  * 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.
  */
 
 #ifndef INCLUDE_DART_API_H_
 #define INCLUDE_DART_API_H_
 
 /** \mainpage Dart Embedding API Reference
@@ skipping 373 matching lines @@
  * argument handles refer to the same object.
  *
  * \param obj1 An object to be compared.
  * \param obj2 An object to be compared.
  *
  * \return True if the objects are identically equal.  False otherwise.
  */
 DART_EXPORT bool Dart_IdentityEquals(Dart_Handle obj1, Dart_Handle obj2);
 
 /**
+ * Returns a hash code for the argument. The hash code of objects that are equal
+ * according to Dart_IdentityEquals will return the same hash code, but the hash
+ * codes of non-equal objects are not necessarily distinct.
+ *
+ * \param obj An object for which to derive a hash code.
+ *
+ * \return A hash code for the parameter.
+ */
+DART_EXPORT uint64_t Dart_IdentityHash(Dart_Handle obj);
+
+/**
  * Allocates a handle in the current scope from a persistent handle.
  */
 DART_EXPORT Dart_Handle Dart_HandleFromPersistent(Dart_PersistentHandle object);
 
 /**
  * Allocates a handle in the current scope from a weak persistent handle.
  */
 DART_EXPORT Dart_Handle Dart_HandleFromWeakPersistent(
     Dart_WeakPersistentHandle object);
 
@@ skipping 2224 matching lines @@
  * NOTE: If multiple callbacks with the same name are registered, only the
  * last callback registered will be remembered.
  */
 DART_EXPORT void Dart_RegisterRootServiceRequestCallback(
     const char* name,
     Dart_ServiceRequestCallback callback,
     void* user_data);
 
 
 #endif  /* INCLUDE_DART_API_H_ */  /* NOLINT */