A MessageObject is a purely internal object to hold information about

src/objects-debug.cc

Index: src/objects-debug.cc
diff --git a/src/objects-debug.cc b/src/objects-debug.cc
index f9c57e696078da0a7e2ab6e644f68ddf0f762861..c8246c86c9615b5b247fd4f674434caae1a45fab 100644
--- a/src/objects-debug.cc
+++ b/src/objects-debug.cc
@@ -158,6 +158,9 @@ void HeapObject::HeapObjectVerify() {
     case SHARED_FUNCTION_INFO_TYPE:
       SharedFunctionInfo::cast(this)->SharedFunctionInfoVerify();
       break;
+    case JS_MESSAGE_OBJECT_TYPE:
+      JSMessageObject::cast(this)->JSMessageObjectVerify();
+      break;
 
 #define MAKE_STRUCT_CASE(NAME, Name, name) \
   case NAME##_TYPE:                        \
@@ -296,6 +299,19 @@ void JSValue::JSValueVerify() {
 }
 
 
+void JSMessageObject::JSMessageObjectVerify() {
+  CHECK(IsJSMessageObject());
+  CHECK(type()->IsString());
+  CHECK(arguments()->IsJSArray());
+  VerifyObjectField(kStartPositionOffset);
+  VerifyObjectField(kEndPositionOffset);
+  VerifyObjectField(kArgumentsOffset);
+  VerifyObjectField(kScriptOffset);
+  VerifyObjectField(kStackTraceOffset);
+  VerifyObjectField(kStackFramesOffset);
+}
+
+
 void String::StringVerify() {
   CHECK(IsString());
   CHECK(length() >= 0 && length() <= Smi::kMaxValue);