Preserve the contents of Dart strings with unmatched surrogate halfs by avoiding a UTF16 -> UTF8

runtime/platform/json.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_JSON_H_
 #define PLATFORM_JSON_H_
 
 #include "vm/allocation.h"
 #include "vm/globals.h"
 
@@ skipping 108 matching lines @@
 
 // TextBuffer maintains a dynamic character buffer with a printf-style way to
 // append text.
 class TextBuffer : ValueObject {
  public:
   explicit TextBuffer(intptr_t buf_size);
   ~TextBuffer();
 
   intptr_t Printf(const char* format, ...) PRINTF_ATTRIBUTE(2, 3);
   void AddChar(char ch);
-  void AddUTF8(uint32_t ch);
-  void AddEscapedChar(uint32_t ch);
+  void EscapeAndAddCodeUnit(uint32_t cu);
   void AddString(const char* s);
   void AddEscapedString(const char* s);
 
   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();
 
  private:
   void EnsureCapacity(intptr_t len);
   char* buf_;
   intptr_t buf_size_;
   intptr_t msg_len_;
 };
 
 }  // namespace dart
 
 #endif  // PLATFORM_JSON_H_