Make sure builtins preserve guarantees about empty element array prototypes.

src/builtins.cc

 // Copyright 2012 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/api.h"
 #include "src/api-natives.h"
 #include "src/arguments.h"
 #include "src/base/once.h"
@@ skipping 216 matching lines @@
     if (args == NULL || array->HasFastObjectElements()) return elms;
   } else if (map == heap->fixed_cow_array_map()) {
     elms = JSObject::EnsureWritableFastElements(array);
     if (args == NULL || array->HasFastObjectElements()) return elms;
   } else if (map == heap->fixed_double_array_map()) {
     if (args == NULL) return elms;
   } else {
     return MaybeHandle<FixedArrayBase>();
   }
 
+  // Adding elements to the array prototype would break code that makes sure
+  // it has no elements. Handle that elsewhere.
+  if (array->GetIsolate()->is_initial_array_prototype(*array)) {
+    return MaybeHandle<FixedArrayBase>();
+  }
+
   // Need to ensure that the arguments passed in args can be contained in
   // the array.
   int args_length = args->length();
   if (first_added_arg >= args_length) return handle(array->elements(), isolate);
 
   ElementsKind origin_kind = array->map()->elements_kind();
   DCHECK(!IsFastObjectElementsKind(origin_kind));
   ElementsKind target_kind = origin_kind;
   {
     DisallowHeapAllocation no_gc;
@@ skipping 1405 matching lines @@
 }
 BUILTIN_LIST_C(DEFINE_BUILTIN_ACCESSOR_C)
 BUILTIN_LIST_A(DEFINE_BUILTIN_ACCESSOR_A)
 BUILTIN_LIST_H(DEFINE_BUILTIN_ACCESSOR_H)
 BUILTIN_LIST_DEBUG_A(DEFINE_BUILTIN_ACCESSOR_A)
 #undef DEFINE_BUILTIN_ACCESSOR_C
 #undef DEFINE_BUILTIN_ACCESSOR_A
 
 
 } }  // namespace v8::internal