Add Dart API method Dart_SetPersistentHandle to enable setting the value of

runtime/include/dart_api.h

Index: runtime/include/dart_api.h
===================================================================
--- runtime/include/dart_api.h	(revision 31439)
+++ runtime/include/dart_api.h	(working copy)
@@ -397,6 +397,20 @@
 DART_EXPORT Dart_PersistentHandle Dart_NewPersistentHandle(Dart_Handle object);
 
 /**
+ * Assign value of local handle to a persistent handle.
+ *
+ * Requires there to be a current isolate.
+ *
+ * \param obj1 A persistent handle whose value needs to be set.
+ * \param obj2 An object whose value needs to be set to the persistent handle.
+ *
+ * \return Success if the persistent handle was set
+ *   Otherwise, returns an error.
+ */
+DART_EXPORT Dart_Handle Dart_SetPersistentHandle(Dart_PersistentHandle obj1,
+                                                 Dart_Handle obj2);
+
+/**
  * Deallocates a persistent handle.
  *
  * Requires there to be a current isolate.