The peer pointer was not being setup at all, fixed that and added a test case

runtime/vm/object.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/object.h"
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "vm/assembler.h"
 #include "vm/cpu.h"
@@ skipping 13417 matching lines @@
 
 
 void OneByteString::SetPeer(const String& str,
                             void* peer,
                             Dart_PeerFinalizer cback) {
   ASSERT(!str.IsNull() && str.IsOneByteString());
   ASSERT(peer != NULL);
   ExternalStringData<uint8_t>* ext_data =
       new ExternalStringData<uint8_t>(NULL, peer, cback);
   AddFinalizer(str, ext_data, OneByteString::Finalize);
+  Isolate::Current()->heap()->SetPeer(str.raw(), peer);
 }
 
 
 void OneByteString::Finalize(Dart_WeakPersistentHandle handle, void* peer) {
   delete reinterpret_cast<ExternalStringData<uint8_t>*>(peer);
   DeleteWeakPersistentHandle(handle);
 }
 
 
 RawTwoByteString* TwoByteString::EscapeSpecialCharacters(const String& str) {
@@ skipping 149 matching lines @@
 
 
 void TwoByteString::SetPeer(const String& str,
                             void* peer,
                             Dart_PeerFinalizer cback) {
   ASSERT(!str.IsNull() && str.IsTwoByteString());
   ASSERT(peer != NULL);
   ExternalStringData<uint16_t>* ext_data =
       new ExternalStringData<uint16_t>(NULL, peer, cback);
   AddFinalizer(str, ext_data, TwoByteString::Finalize);
+  Isolate::Current()->heap()->SetPeer(str.raw(), peer);
 }
 
 
 void TwoByteString::Finalize(Dart_WeakPersistentHandle handle, void* peer) {
   delete reinterpret_cast<ExternalStringData<uint16_t>*>(peer);
   DeleteWeakPersistentHandle(handle);
 }
 
 
 RawExternalOneByteString* ExternalOneByteString::New(
@@ skipping 1155 matching lines @@
 }
 
 
 void MirrorReference::PrintToJSONStream(JSONStream* stream, bool ref) const {
   stream->OpenObject();
   stream->CloseObject();
 }
 
 
 }  // namespace dart