Report promise reject with no handler (behind a flag).

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index e2321974d3fb03c7b92fc726f3f1fa2411dc1d61..00cf8ff86615f3fd4c56dcab0d28124647e91171 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -7655,6 +7655,9 @@ class JSMessageObject: public JSObject {
   // [arguments]: the arguments for formatting the error message.
   DECL_ACCESSORS(arguments, JSArray)
 
+  // [promise]: the promise (if any) associated to the error message.
+  DECL_ACCESSORS(promise, Object)
+
   // [script]: the script from which the error message originated.
   DECL_ACCESSORS(script, Object)
 
@@ -7678,7 +7681,8 @@ class JSMessageObject: public JSObject {
   // Layout description.
   static const int kTypeOffset = JSObject::kHeaderSize;
   static const int kArgumentsOffset = kTypeOffset + kPointerSize;
-  static const int kScriptOffset = kArgumentsOffset + kPointerSize;
+  static const int kPromiseOffset = kArgumentsOffset + kPointerSize;
+  static const int kScriptOffset = kPromiseOffset + kPointerSize;
   static const int kStackFramesOffset = kScriptOffset + kPointerSize;
   static const int kStartPositionOffset = kStackFramesOffset + kPointerSize;
   static const int kEndPositionOffset = kStartPositionOffset + kPointerSize;