[Interpreter] Implement ForInStep directly.

src/interpreter/interpreter.cc

 // Copyright 2015 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/interpreter/interpreter.h"
 
 #include "src/ast/prettyprinter.h"
 #include "src/code-factory.h"
 #include "src/compiler.h"
 #include "src/factory.h"
@@ skipping 1815 matching lines @@
   __ SetAccumulator(result);
   __ Dispatch();
 }
 
 
 // ForInStep <index>
 //
 // Increments the loop counter in register |index| and stores the result
 // in the accumulator.
 void Interpreter::DoForInStep(InterpreterAssembler* assembler) {
-  // TODO(oth): Implement directly rather than making a runtime call.
   Node* index_reg = __ BytecodeOperandReg(0);
   Node* index = __ LoadRegister(index_reg);
-  Node* context = __ GetContext();
-  Node* result = __ CallRuntime(Runtime::kForInStep, context, index);
+  Node* one = __ SmiConstant(Smi::FromInt(1));
+  Node* result = __ SmiAdd(index, one);
   __ SetAccumulator(result);
   __ Dispatch();
 }
 
 }  // namespace interpreter
 }  // namespace internal
 }  // namespace v8