Do not flatten the string, but just write it out....

src/api.cc

 // Copyright 2009 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 2428 matching lines @@
   return i;
 }
 
 
 int String::Write(uint16_t* buffer, int start, int length) const {
   if (IsDeadCheck("v8::String::Write()")) return 0;
   LOG_API("String::Write");
   ENTER_V8;
   ASSERT(start >= 0 && length >= -1);
   i::Handle<i::String> str = Utils::OpenHandle(this);
-  // Flatten the string for efficiency.  This applies whether we are
-  // using StringInputBuffer or Get(i) to access the characters.
-  str->TryFlattenIfNotFlat();
   int end = length;
   if ( (length == -1) || (length > str->length() - start) )
     end = str->length() - start;
   if (end < 0) return 0;
-  write_input_buffer.Reset(start, *str);
-  int i;
-  for (i = 0; i < end; i++)
-    buffer[i] = write_input_buffer.GetNext();
-  if (length == -1 || i < length)
-    buffer[i] = '\0';
-  return i;
+  i::String::WriteToFlat(*str, buffer, start, end);
+  if (length == -1 || end < length)
+    buffer[end] = '\0';
+  return end;
 }
 
 
 bool v8::String::IsExternal() const {
   EnsureInitialized("v8::String::IsExternal()");
   i::Handle<i::String> str = Utils::OpenHandle(this);
   return i::StringShape(*str).IsExternalTwoByte();
 }
 
 
@@ skipping 1293 matching lines @@
 
 
 char* HandleScopeImplementer::Iterate(ObjectVisitor* v, char* storage) {
   HandleScopeImplementer* thread_local =
       reinterpret_cast<HandleScopeImplementer*>(storage);
   thread_local->IterateThis(v);
   return storage + ArchiveSpacePerThread();
 }
 
 } }  // namespace v8::internal