Introduce legacy const slots in correct context.

src/runtime/runtime-scopes.cc

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/v8.h"
 
 #include "src/accessors.h"
 #include "src/arguments.h"
 #include "src/frames-inl.h"
 #include "src/runtime/runtime-utils.h"
@@ skipping 288 matching lines @@
   }
 
   PropertyAttributes attr =
       static_cast<PropertyAttributes>(DONT_DELETE | READ_ONLY);
 
   // Strict mode handling not needed (legacy const is disallowed in strict
   // mode).
 
   // The declared const was configurable, and may have been deleted in the
   // meanwhile. If so, re-introduce the variable in the context extension.
-  DCHECK(context_arg->has_extension());
   if (attributes == ABSENT) {
-    holder = handle(context_arg->extension(), isolate);
+    Handle<Context> declaration_context(context_arg->declaration_context());
+    DCHECK(declaration_context->has_extension());
+    holder = handle(declaration_context->extension(), isolate);
+    CHECK(holder->IsJSObject());
   } else {
     // For JSContextExtensionObjects, the initializer can be run multiple times
     // if in a for loop: for (var i = 0; i < 2; i++) { const x = i; }. Only the
     // first assignment should go through. For JSGlobalObjects, additionally any
     // code can run in between that modifies the declared property.
     DCHECK(holder->IsJSGlobalObject() || holder->IsJSContextExtensionObject());
 
     LookupIterator it(holder, name, LookupIterator::HIDDEN_SKIP_INTERCEPTOR);
     Maybe<PropertyAttributes> maybe = JSReceiver::GetPropertyAttributes(&it);
     if (!maybe.has_value) return isolate->heap()->exception();
@@ skipping 747 matching lines @@
   return Smi::FromInt(frame->GetArgumentsLength());
 }
 
 
 RUNTIME_FUNCTION(RuntimeReference_Arguments) {
   SealHandleScope shs(isolate);
   return __RT_impl_Runtime_GetArgumentsProperty(args, isolate);
 }
 }
 }  // namespace v8::internal