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

runtime/vm/object.h

Index: runtime/vm/object.h
===================================================================
--- runtime/vm/object.h	(revision 26534)
+++ runtime/vm/object.h	(working copy)
@@ -4608,7 +4608,6 @@
                           void* peer,
                           Dart_PeerFinalizer cback) const;
 
-
   // Creates a new String object from a C string that is assumed to contain
   // UTF-8 encoded characters and '\0' is considered a termination character.
   // TODO(7123) - Rename this to FromCString(....).
@@ -4808,6 +4807,12 @@
                                               intptr_t length,
                                               Heap::Space space);
 
+  static void SetPeer(const String& str,
+                      void* peer,
+                      Dart_PeerFinalizer cback);
+
+  static void Finalize(Dart_WeakPersistentHandle handle, void* peer);
+
   static const ClassId kClassId = kOneByteStringCid;
 
   static RawOneByteString* null() {
@@ -4890,6 +4895,12 @@
                                      const String& str,
                                      Heap::Space space);
 
+  static void SetPeer(const String& str,
+                      void* peer,
+                      Dart_PeerFinalizer cback);
+
+  static void Finalize(Dart_WeakPersistentHandle handle, void* peer);
+
   static RawTwoByteString* null() {
     return reinterpret_cast<RawTwoByteString*>(Object::null());
   }