Add Dart_IdentityHash to the embedding API.

runtime/include/dart_api.h

Index: runtime/include/dart_api.h
diff --git a/runtime/include/dart_api.h b/runtime/include/dart_api.h
index 5648eb60ab981addcdf3003878016d14d400ca1e..77da50cc79b2f91f9bea4f08c78ec1f51f59f56c 100755
--- a/runtime/include/dart_api.h
+++ b/runtime/include/dart_api.h
@@ -391,6 +391,17 @@ DART_EXPORT Dart_Handle Dart_ToString(Dart_Handle object);
 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);