| Index: src/builtins.cc
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| diff --git a/src/builtins.cc b/src/builtins.cc
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| index 62e4372d0e7b579d5fd726becec9fc80aa065ed4..538b657f2fe9937bd2315aba2ac56d3fded14de0 100644
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| --- a/src/builtins.cc
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| +++ b/src/builtins.cc
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| @@ -312,6 +312,7 @@ static inline MaybeObject* EnsureJSArrayWithWritableFastElements(
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|    if (!receiver->IsJSArray()) return NULL;
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|    JSArray* array = JSArray::cast(receiver);
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|    if (array->map()->is_observed()) return NULL;
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| +  if (!array->map()->is_extensible()) return NULL;
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|    HeapObject* elms = array->elements();
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|    Map* map = elms->map();
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|    if (map == heap->fixed_array_map()) {
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| 
 |