Add debug-stepnext test for for-let loops

test/mjsunit/es6/debug-stepnext-for.js

 // Copyright 2014 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 --harmony
 
 Debug = debug.Debug;
 var break_count = 0
 var exception = null;
 var log = []
@@ skipping 40 matching lines @@
   }
 
   for (var i = 0; i < 3; i++) {  // Break k
     s += a[i];                   // Break K
   }
 
   for (j = 0; j < 3; j++) {      // Break l
     s += a[j];                   // Break L
   }
 
-  // TODO(yangguo): add test case for for-let.
+  for (let i = 0; i < 3; i++) {  // Break m
+    s += a[i];                   // Break M
+  }
 }                                // Break y
 
 function listener(event, exec_state, event_data, data) {
   if (event != Debug.DebugEvent.Break) return;
   try {
     var line = exec_state.frame(0).sourceLineText();
     var col = exec_state.frame(0).sourceColumn();
     print(line);
     var match = line.match(/\/\/ Break (\w)$/);
     assertEquals(2, match.length);
@@ skipping 28 matching lines @@
   // For-of-var: next(), body, next(), body, ...
   "h16","H4","h16","H4","h16","H4","h16",
   // For-of: next(), body, next(), body, ...
   "i12","I4","i12","I4","i12","I4","i12",
   // For-of-let: next(), body, next(), ...
   "j16","J4","j16","J4","j16","J4","j16",
   // For-var: var decl, condition, body, next, condition, body, ...
   "k7","k20","K4","k23","k20","K4","k23","k20","K4","k23","k20",
   // For: init, condition, body, next, condition, body, ...
   "l7","l16","L4","l19","l16","L4","l19","l16","L4","l19","l16",
+  // For-let: init, condition, body, next, condition, body, ...
+  "m7","m20","M4","m23","m20","M4","m23","m20","M4","m23","m20",
   // Exit.
   "y0","z0",
 ]
 print("expected:\n"+ JSON.stringify(expected));
 
 assertArrayEquals(expected, log);
-assertEquals(48, s);
+assertEquals(54, s);
 assertNull(exception);