Setup a peer for read only strings that are not externalized in

runtime/vm/dart_api_impl.h

Index: runtime/vm/dart_api_impl.h
===================================================================
--- runtime/vm/dart_api_impl.h	(revision 26385)
+++ runtime/vm/dart_api_impl.h	(working copy)
@@ -191,10 +191,9 @@
   static void InitHandles();
 
   // Helper function to get the peer value of an external string object.
-  static bool ExternalStringGetPeerHelper(Dart_NativeArguments args,
-                                          int arg_index,
-                                          void** peer);
-  static bool ExternalStringGetPeerHelper(Dart_Handle object, void** peer);
+  static bool StringGetPeerHelper(Dart_NativeArguments args,
+                                  int arg_index,
+                                  void** peer);
 
   // Helper function to set the return value of native functions.
   static void SetReturnValue(NativeArguments* args, Dart_Handle retval) {