A MessageObject is a purely internal object to hold information about

src/objects-printer.cc

Index: src/objects-printer.cc
diff --git a/src/objects-printer.cc b/src/objects-printer.cc
index 9f05b0f62d295af993cd7627a6f978248de3619b..237358dcc7191ab93534aa44b6e78af639dfbce7 100644
--- a/src/objects-printer.cc
+++ b/src/objects-printer.cc
@@ -151,6 +151,9 @@ void HeapObject::HeapObjectPrint(FILE* out) {
     case SHARED_FUNCTION_INFO_TYPE:
       SharedFunctionInfo::cast(this)->SharedFunctionInfoPrint(out);
       break;
+    case JS_MESSAGE_OBJECT_TYPE:
+      JSMessageObject::cast(this)->JSMessageObjectPrint(out);
+      break;
     case JS_GLOBAL_PROPERTY_CELL_TYPE:
       JSGlobalPropertyCell::cast(this)->JSGlobalPropertyCellPrint(out);
       break;
@@ -396,6 +399,7 @@ static const char* TypeToString(InstanceType type) {
     case JS_GLOBAL_PROXY_TYPE: return "JS_GLOBAL_PROXY";
     case PROXY_TYPE: return "PROXY";
     case LAST_STRING_TYPE: return "LAST_STRING_TYPE";
+    case JS_MESSAGE_OBJECT_TYPE: return "JS_MESSAGE_OBJECT_TYPE";
 #define MAKE_STRUCT_CASE(NAME, Name, name) case NAME##_TYPE: return #NAME;
   STRUCT_LIST(MAKE_STRUCT_CASE)
 #undef MAKE_STRUCT_CASE
@@ -466,6 +470,24 @@ void JSValue::JSValuePrint(FILE* out) {
 }
 
 
+void JSMessageObject::JSMessageObjectPrint(FILE* out) {
+  HeapObject::PrintHeader(out, "JSMessageObject");
+  PrintF(out, " - type: ");
+  type()->ShortPrint(out);
+  PrintF(out, "\n - arguments: ");
+  arguments()->ShortPrint(out);
+  PrintF(out, "\n - start_position: %d", start_position());
+  PrintF(out, "\n - end_position: %d", end_position());
+  PrintF(out, "\n - script: ");
+  script()->ShortPrint(out);
+  PrintF(out, "\n - stack_trace: ");
+  stack_trace()->ShortPrint(out);
+  PrintF(out, "\n - stack_frames: ");
+  stack_frames()->ShortPrint(out);
+  PrintF(out, "\n");
+}
+
+
 void String::StringPrint(FILE* out) {
   if (StringShape(this).IsSymbol()) {
     PrintF(out, "#");