Stay in fast enum case if the empty_slow_element_dictionary is used.

src/arm/macro-assembler-arm.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * 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
@@ skipping 3908 matching lines @@
 
   // For all objects but the receiver, check that the cache is empty.
   EnumLength(r3, r1);
   cmp(r3, Operand(Smi::FromInt(0)));
   b(ne, call_runtime);
 
   bind(&start);
 
   // Check that there are no elements. Register r2 contains the current JS
   // object we've reached through the prototype chain.
+  Label no_elements;
   ldr(r2, FieldMemOperand(r2, JSObject::kElementsOffset));
   cmp(r2, empty_fixed_array_value);
+  b(eq, &no_elements);
+
+  // Second chance, the object may be using the empty slow element dictionary.
+  CompareRoot(r2, Heap::kEmptySlowElementDictionaryRootIndex);
   b(ne, call_runtime);
 
+  bind(&no_elements);
   ldr(r2, FieldMemOperand(r1, Map::kPrototypeOffset));
   cmp(r2, null_value);
   b(ne, &next);
 }
 
 
 void MacroAssembler::TestJSArrayForAllocationMemento(
     Register receiver_reg,
     Register scratch_reg,
     Label* no_memento_found) {
@@ skipping 128 matching lines @@
 void CodePatcher::EmitCondition(Condition cond) {
   Instr instr = Assembler::instr_at(masm_.pc_);
   instr = (instr & ~kCondMask) | cond;
   masm_.emit(instr);
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_ARM