Do not allow external strings in old pointer space.

src/mark-compact.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 2725 matching lines @@
 void MarkCompactCollector::MigrateObject(Address dst,
                                          Address src,
                                          int size,
                                          AllocationSpace dest) {
   HEAP_PROFILE(heap(), ObjectMoveEvent(src, dst));
   // TODO(hpayer): Replace that check with an assert.
   CHECK(dest != LO_SPACE && size <= Page::kMaxNonCodeHeapObjectSize);
   if (dest == OLD_POINTER_SPACE) {
     // TODO(hpayer): Replace this check with an assert.
     HeapObject* heap_object = HeapObject::FromAddress(src);
-    CHECK(heap_object->IsExternalString() ||
-          heap_->TargetSpace(heap_object) == heap_->old_pointer_space());
+    CHECK(heap_->TargetSpace(heap_object) == heap_->old_pointer_space());
     Address src_slot = src;
     Address dst_slot = dst;
     ASSERT(IsAligned(size, kPointerSize));
 
     for (int remaining = size / kPointerSize; remaining > 0; remaining--) {
       Object* value = Memory::Object_at(src_slot);
 
       Memory::Object_at(dst_slot) = value;
 
       if (heap_->InNewSpace(value)) {
@@ skipping 1580 matching lines @@
   while (buffer != NULL) {
     SlotsBuffer* next_buffer = buffer->next();
     DeallocateBuffer(buffer);
     buffer = next_buffer;
   }
   *buffer_address = NULL;
 }
 
 
 } }  // namespace v8::internal