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

src/api.h

Index: src/api.h
diff --git a/src/api.h b/src/api.h
index 9aed5dd4e971e859790fcb02e9d1117baa5bda17..1dc894701031db688538b1df73161c894c9aee21 100644
--- a/src/api.h
+++ b/src/api.h
@@ -232,6 +232,8 @@ class Utils {
 
   static inline Local<Message> MessageToLocal(
       v8::internal::Handle<v8::internal::Object> obj);
+  static inline Local<Promise> PromiseToLocal(
+      v8::internal::Handle<v8::internal::JSObject> obj);
   static inline Local<StackTrace> StackTraceToLocal(
       v8::internal::Handle<v8::internal::JSArray> obj);
   static inline Local<StackFrame> StackFrameToLocal(
@@ -355,6 +357,7 @@ MAKE_TO_LOCAL(ToLocal, SignatureInfo, Signature)
 MAKE_TO_LOCAL(AccessorSignatureToLocal, FunctionTemplateInfo, AccessorSignature)
 MAKE_TO_LOCAL(ToLocal, TypeSwitchInfo, TypeSwitch)
 MAKE_TO_LOCAL(MessageToLocal, Object, Message)
+MAKE_TO_LOCAL(PromiseToLocal, JSObject, Promise)
 MAKE_TO_LOCAL(StackTraceToLocal, JSArray, StackTrace)
 MAKE_TO_LOCAL(StackFrameToLocal, JSObject, StackFrame)
 MAKE_TO_LOCAL(NumberToLocal, Object, Number)