[ES6 modules] Return previous error when an instantiation is reattempt.

third_party/WebKit/Source/core/testing/DummyModulator.h

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef DummyModulator_h
 #define DummyModulator_h
 
 #include "bindings/core/v8/ScriptModule.h"
 #include "core/dom/Modulator.h"
 #include "platform/heap/Handle.h"
@@ skipping 34 matching lines @@
                    ModuleGraphLevel,
                    SingleModuleClient*) override;
   ModuleScript* GetFetchedModuleScript(const KURL&) override;
   void FetchNewSingleModule(const ModuleScriptFetchRequest&,
                             ModuleGraphLevel,
                             ModuleScriptLoaderClient*) override;
   ScriptModule CompileModule(const String& script,
                              const String& url_str,
                              AccessControlStatus) override;
   ScriptValue InstantiateModule(ScriptModule) override;
+  ScriptValue GetInstantiationError(const ModuleScript*) override;
   Vector<String> ModuleRequestsFromScriptModule(ScriptModule) override;
   void ExecuteModule(const ModuleScript*) override;
 };
 
 }  // namespace blink
 
 #endif  // DummyModulator_h