Reduce copying in sending service responses.

runtime/platform/text_buffer.h

 // 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.
 
 #ifndef PLATFORM_TEXT_BUFFER_H_
 #define PLATFORM_TEXT_BUFFER_H_
 
 #include "vm/allocation.h"
 #include "vm/globals.h"
 
@@ skipping 16 matching lines @@
   void AddRaw(const uint8_t* buffer,
               intptr_t buffer_length);
 
   void Clear();
 
   char* buf() { return buf_; }
   intptr_t length() { return msg_len_; }
 
   // Steal ownership of the buffer pointer.
   // NOTE: TextBuffer is empty afterwards.
-  const char* Steal();
+  char* Steal();
 
  private:
   void EnsureCapacity(intptr_t len);
   char* buf_;
   intptr_t buf_size_;
   intptr_t msg_len_;
 };
 
 }  // namespace dart
 
 #endif  // PLATFORM_TEXT_BUFFER_H_