Make generators non-constructable.

src/runtime/runtime-generator.cc

Index: src/runtime/runtime-generator.cc
diff --git a/src/runtime/runtime-generator.cc b/src/runtime/runtime-generator.cc
index 926cd3ce2d6d107e6b94bb2f3caa44cd1bd6cdc9..54894a013a624d287fe4efe03e04bbc678410527 100644
--- a/src/runtime/runtime-generator.cc
+++ b/src/runtime/runtime-generator.cc
@@ -22,11 +22,8 @@ RUNTIME_FUNCTION(Runtime_CreateJSGeneratorObject) {
   RUNTIME_ASSERT(function->shared()->is_generator());
 
   Handle<JSGeneratorObject> generator;
-  if (frame->IsConstructor()) {
-    generator = handle(JSGeneratorObject::cast(frame->receiver()));
-  } else {
-    generator = isolate->factory()->NewJSGeneratorObject(function);
-  }
+  DCHECK(!frame->IsConstructor());
+  generator = isolate->factory()->NewJSGeneratorObject(function);
   generator->set_function(*function);
   generator->set_context(Context::cast(frame->context()));
   generator->set_receiver(frame->receiver());