[liveedit] fix stepping after replacing bytecode.

test/mjsunit/debug-liveedit-stepin.js

 // 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.
 
 // Flags: --expose-debug-as debug
 
 Debug = debug.Debug
 
 function BestEditor() {
-  var best_editor = "Emacs";
-  return best_editor;
+  return 'Emacs';
 }
 
 var exception = null;
 var results = [];
 var log = []
 
 function listener(event, exec_state, event_data, data) {
   if (event != Debug.DebugEvent.Break) return;
   try {
     var source_line = event_data.sourceLineText();
@@ skipping 33 matching lines @@
 results.push(BestEditor());
 results.push(BestEditor());
 Debug.setListener(null);
 
 assertNull(exception);
 assertEquals(["Emacs", "Eclipse", "Vim"], results);
 print(JSON.stringify(log, 1));
 assertEquals([
   "debugger;",
   "results.push(BestEditor());",
-  "  var best_editor = \"Emacs\";",
-  "  return best_editor;","}",
+  "  return 'Emacs';","}",
   "results.push(BestEditor());",
   "results.push(BestEditor());",
-  "  var best_editor = \"Emacs\";",
-  "  return best_editor;",
-  "  var best_editor = \"Eclipse\";",
-  "  return best_editor;","}",
+  "  return 'Emacs';",
+  "  return 'Eclipse';","}",
   "results.push(BestEditor());",
   "results.push(BestEditor());",
-  "  var best_editor = \"Eclipse\";",
-  "  return best_editor;",
-  "  var best_editor = \"Vim\";",
-  "  return best_editor;",
+  "  return 'Eclipse';",
+  "  return 'Vim';",
   "}","results.push(BestEditor());",
   "Debug.setListener(null);"
 ], log);