new classes: no longer experimental.

src/runtime/runtime-classes.cc

Index: src/runtime/runtime-classes.cc
diff --git a/src/runtime/runtime-classes.cc b/src/runtime/runtime-classes.cc
index ea641d3faf781a7ed033bec80678dc667f5df617..df428f43aea083194eae084f7e24cc5e67efc124 100644
--- a/src/runtime/runtime-classes.cc
+++ b/src/runtime/runtime-classes.cc
@@ -414,6 +414,19 @@ RUNTIME_FUNCTION(Runtime_StoreKeyedToSuper_Sloppy) {
 }
 
 
+RUNTIME_FUNCTION(Runtime_HandleStepInForDerivedConstructors) {
+  HandleScope scope(isolate);
+  DCHECK(args.length() == 1);
+  CONVERT_ARG_HANDLE_CHECKED(JSFunction, function, 0);
+  Debug* debug = isolate->debug();
+  // Handle stepping into constructors if step into is active.
+  if (debug->StepInActive()) {
+    debug->HandleStepIn(function, Handle<Object>::null(), 0, true);
+  }
+  return *isolate->factory()->undefined_value();
+}
+
+
 RUNTIME_FUNCTION(RuntimeReference_DefaultConstructorCallSuper) {
   UNREACHABLE();
   return nullptr;