Performance improvement for the Dart VM runtime method

runtime/vm/raw_object.h

Index: runtime/vm/raw_object.h
===================================================================
--- runtime/vm/raw_object.h	(revision 11532)
+++ runtime/vm/raw_object.h	(working copy)
@@ -1123,10 +1123,16 @@
 };
 
 
+DART_EXTERN_C {
+  Dart_Handle Dart_ExternalStringGetPeer(Dart_Handle, void**);
+}
+
+
 class RawExternalOneByteString : public RawString {
   RAW_HEAP_OBJECT_IMPLEMENTATION(ExternalOneByteString);
 
   ExternalStringData<uint8_t>* external_data_;
+  friend Dart_Handle Dart_ExternalStringGetPeer(Dart_Handle, void**);
 };
 
 
@@ -1134,6 +1140,7 @@
   RAW_HEAP_OBJECT_IMPLEMENTATION(ExternalTwoByteString);
 
   ExternalStringData<uint16_t>* external_data_;
+  friend Dart_Handle Dart_ExternalStringGetPeer(Dart_Handle, void**);
 };
 
 
@@ -1141,6 +1148,7 @@
   RAW_HEAP_OBJECT_IMPLEMENTATION(ExternalFourByteString);
 
   ExternalStringData<uint32_t>* external_data_;
+  friend Dart_Handle Dart_ExternalStringGetPeer(Dart_Handle, void**);
 };