Trigger exception debug event for promises at the throw site.

src/debug.h

Index: src/debug.h
diff --git a/src/debug.h b/src/debug.h
index da59526e6ae02a04c188068dca668e47ddbf4063..b76c512c9c6820bcde378a4f7dac2cdb4b7c7506 100644
--- a/src/debug.h
+++ b/src/debug.h
@@ -235,6 +235,12 @@ class Debug {
   void FloodHandlerWithOneShot();
   void ChangeBreakOnException(ExceptionBreakType type, bool enable);
   bool IsBreakOnException(ExceptionBreakType type);
+
+  void PromiseHandlePrologue(Handle<JSFunction> promise_getter);
+  void PromiseHandleEpilogue();
+  // Returns a promise if it does not have a reject handler.
+  Handle<Object> GetPromiseForUncaughtException();
+
   void PrepareStep(StepAction step_action,
                    int step_count,
                    StackFrame::Id frame_id);
@@ -541,6 +547,13 @@ class Debug {
   bool break_on_exception_;
   bool break_on_uncaught_exception_;
 
+  // When a promise is being resolved, we may want to trigger a debug event for
+  // the case we catch a throw.  For this purpose we remember the try-catch
+  // handler address that would catch the exception.  We also hold onto a
+  // closure that returns a promise if the exception is considered uncaught.
+  StackHandler* current_promise_catch_handler_;
+  MaybeHandle<JSFunction> current_promise_getter_;
+
   // Per-thread data.
   class ThreadLocal {
    public:
@@ -777,9 +790,7 @@ class Debugger {
   MUST_USE_RESULT MaybeHandle<Object> MakeScriptCollectedEvent(int id);
 
   void OnDebugBreak(Handle<Object> break_points_hit, bool auto_continue);
-  void OnException(Handle<Object> exception,
-                   bool uncaught,
-                   Handle<Object> promise = Handle<Object>::null());
+  void OnException(Handle<Object> exception, bool uncaught);
   void OnBeforeCompile(Handle<Script> script);
 
   enum AfterCompileFlags {