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

runtime/vm/json_stream.h

Index: runtime/vm/json_stream.h
diff --git a/runtime/vm/json_stream.h b/runtime/vm/json_stream.h
index 1e843680a8e79610fcb3b556e76691058911a1e7..8b4fc1e842e0753c9c845fc00a8c78652c826d73 100644
--- a/runtime/vm/json_stream.h
+++ b/runtime/vm/json_stream.h
@@ -93,13 +93,14 @@ class JSONStream : ValueObject {
   void PrintValue(const DebuggerEvent* event);
   void PrintValue(Metric* metric);
   void PrintValue(Isolate* isolate, bool ref = true);
+  bool PrintValueStr(const String& s, intptr_t limit);
 
   void PrintPropertyBool(const char* name, bool b);
   void PrintProperty(const char* name, intptr_t i);
   void PrintProperty64(const char* name, int64_t i);
   void PrintProperty(const char* name, double d);
   void PrintProperty(const char* name, const char* s);
-  void PrintProperty(const char* name, const char* s, intptr_t len);
+  bool PrintPropertyStr(const char* name, const String& s, intptr_t limit);
   void PrintPropertyNoEscape(const char* name, const char* s);
   void PrintfProperty(const char* name, const char* format, ...)
   PRINTF_ATTRIBUTE(3, 4);
@@ -113,6 +114,7 @@ class JSONStream : ValueObject {
   void PrintCommaIfNeeded();
   bool NeedComma();
 
+  bool AddDartString(const String& s, intptr_t limit);
   void AddEscapedUTF8String(const char* s);
   void AddEscapedUTF8String(const char* s, intptr_t len);
 
@@ -163,8 +165,10 @@ class JSONObject : public ValueObject {
   void AddProperty(const char* name, const char* s) const {
     stream_->PrintProperty(name, s);
   }
-  void AddProperty(const char* name, const char* s, intptr_t len) const {
-    stream_->PrintProperty(name, s, len);
+  bool AddPropertyStr(const char* name,
+                      const String& s,
+                      intptr_t limit = -1) const {
+    return stream_->PrintPropertyStr(name, s, limit);
   }
   void AddPropertyNoEscape(const char* name, const char* s) const {
     stream_->PrintPropertyNoEscape(name, s);