Allow stepping into Promise handlers.

src/debug.cc

Index: src/debug.cc
diff --git a/src/debug.cc b/src/debug.cc
index 3029adba9d53403677ac35ed4ce05ba2ac7dfde3..f5c2cbb2d7d504662ac7b40039e179720fb79da2 100644
--- a/src/debug.cc
+++ b/src/debug.cc
@@ -1199,6 +1199,9 @@ void Debug::ClearAllBreakPoints() {
 
 void Debug::FloodWithOneShot(Handle<JSFunction> function,
                              BreakLocatorType type) {
+  // Do not ever break in native functions.
+  if (function->IsFromNativeScript()) return;
+
   PrepareForBreakPoints();
 
   // Make sure the function is compiled and has set up the debug info.
@@ -1249,7 +1252,7 @@ void Debug::FloodWithOneShotGeneric(Handle<JSFunction> function,
     FloodBoundFunctionWithOneShot(function);
   } else if (function->shared()->is_default_constructor()) {
     FloodDefaultConstructorWithOneShot(function);
-  } else if (!function->IsFromNativeScript()) {
+  } else {
     Isolate* isolate = function->GetIsolate();
     // Don't allow step into functions in the native context.
     if (function->shared()->code() ==