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

runtime/platform/json.h

Index: runtime/platform/json.h
diff --git a/runtime/platform/json.h b/runtime/platform/json.h
index 36c7d5d123b3f1812a416efd409d4438d0590969..35e975f05a891cc0930c5579d3c50ac0b80a4185 100644
--- a/runtime/platform/json.h
+++ b/runtime/platform/json.h
@@ -126,8 +126,7 @@ class TextBuffer : ValueObject {
 
   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);