Allow externalizing strings in old pointer space.

src/heap-inl.h

Index: src/heap-inl.h
diff --git a/src/heap-inl.h b/src/heap-inl.h
index d42e1cfc1164f396fe0b782d93754ef8f3510e42..09d754ff82164aa961f6dceba941a0db3460a996 100644
--- a/src/heap-inl.h
+++ b/src/heap-inl.h
@@ -418,7 +418,7 @@ AllocationSpace Heap::TargetSpaceId(InstanceType type) {
 }
 
 
-bool Heap::AllowedToBeMigrated(HeapObject* object, AllocationSpace dst) {
+bool Heap::AllowedToBeMigrated(HeapObject* obj, AllocationSpace dst) {
   // Object migration is governed by the following rules:
   //
   // 1) Objects in new-space can be migrated to one of the old spaces
@@ -428,18 +428,22 @@ bool Heap::AllowedToBeMigrated(HeapObject* object, AllocationSpace dst) {
   //    fixed arrays in new-space, old-data-space and old-pointer-space.
   // 4) Fillers (one word) can never migrate, they are skipped by
   //    incremental marking explicitly to prevent invalid pattern.
+  // 5) Short external strings can end up in old pointer space when a cons
+  //    string in old pointer space is made external (String::MakeExternal).
   //
   // Since this function is used for debugging only, we do not place
   // asserts here, but check everything explicitly.
-  if (object->map() == one_pointer_filler_map()) return false;
-  InstanceType type = object->map()->instance_type();
-  MemoryChunk* chunk = MemoryChunk::FromAddress(object->address());
+  if (obj->map() == one_pointer_filler_map()) return false;
+  InstanceType type = obj->map()->instance_type();
+  MemoryChunk* chunk = MemoryChunk::FromAddress(obj->address());
   AllocationSpace src = chunk->owner()->identity();
   switch (src) {
     case NEW_SPACE:
       return dst == src || dst == TargetSpaceId(type);
     case OLD_POINTER_SPACE:
-      return dst == src && (dst == TargetSpaceId(type) || object->IsFiller());
+      return dst == src &&
+          (dst == TargetSpaceId(type) || obj->IsFiller() ||
+          (obj->IsExternalString() && ExternalString::cast(obj)->is_short()));
     case OLD_DATA_SPACE:
       return dst == src && dst == TargetSpaceId(type);
     case CODE_SPACE: