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diff --git a/Source/JavaScriptCore/runtime/LazyClassStructure.h b/Source/JavaScriptCore/runtime/LazyClassStructure.h
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+/*
+ * Copyright (C) 2016 Apple Inc. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY APPLE INC. AND ITS CONTRIBUTORS ``AS IS''
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+ * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS
+ * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
+ * THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include "LazyProperty.h"
+#include "Structure.h"
+
+namespace JSC {
+
+class JSGlobalObject;
+class VM;
+
+class LazyClassStructure {
+    typedef LazyProperty<JSGlobalObject, Structure>::Initializer StructureInitializer;
+    
+public:
+    struct Initializer {
+        Initializer(VM&, JSGlobalObject*, LazyClassStructure&, const StructureInitializer&);
+        
+        // This should be called first or not at all.
+        void setPrototype(JSObject* prototype);
+        
+        // If this is called after setPrototype() then it just sets the structure. If this is
+        // called first then it sets the prototype by extracting it from the structure.
+        void setStructure(Structure*);
+        
+        // Call this last. It's expected that the constructor is initialized to point to the
+        // prototype already. This will automatically set prototype.constructor=constructor.
+        // This will also stuff the constructor into the global object at the given property.
+        // Note that the variant that does not take a property name attempts to deduce it by
+        // casting constructor to either JSFunction or InternalFunction. Also, you can pass
+        // nullptr for the property name, in which case we don't assign the property to the
+        // global object.
+        void setConstructor(PropertyName, JSObject* constructor);
+        void setConstructor(JSObject* constructor);
+        
+        VM& vm;
+        JSGlobalObject* global;
+        LazyClassStructure& classStructure;
+        const StructureInitializer& structureInit;
+        
+        // It's expected that you set these using the set methods above.
+        JSObject* prototype { nullptr };
+        Structure* structure { nullptr };
+        JSObject* constructor { nullptr };
+    };
+    
+    LazyClassStructure()
+    {
+    }
+    
+    template<typename Callback>
+    void initLater(const Callback&);
+    
+    Structure* get(const JSGlobalObject* global) const
+    {
+        ASSERT(!isCompilationThread());
+        return m_structure.get(global);
+    }
+    
+    JSObject* prototype(const JSGlobalObject* global) const
+    {
+        ASSERT(!isCompilationThread());
+        return get(global)->storedPrototypeObject();
+    }
+
+    // Almost as an afterthought, we also support getting the original constructor. This turns
+    // out to be important for ES6 support.
+    JSObject* constructor(const JSGlobalObject* global) const
+    {
+        ASSERT(!isCompilationThread());
+        m_structure.get(global);
+        return m_constructor.get();
+    }
+    
+    Structure* getConcurrently() const
+    {
+        return m_structure.getConcurrently();
+    }
+    
+    JSObject* prototypeConcurrently() const
+    {
+        if (Structure* structure = getConcurrently())
+            return structure->storedPrototypeObject();
+        return nullptr;
+    }
+    
+    JSObject* constructorConcurrently() const
+    {
+        return m_constructor.get();
+    }
+    
+    void visit(SlotVisitor&);
+    
+    void dump(PrintStream&) const;
+
+private:
+    LazyProperty<JSGlobalObject, Structure> m_structure;
+    WriteBarrier<JSObject> m_constructor;
+};
+
+} // namespace JSC