[debugger] fix break locations for assignments and return.

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 73 matching lines @@
 Debug.setListener(null);         // Break z
 
 print("log:\n"+ JSON.stringify(log));
 // The let declaration differs from var in that the loop variable
 // is declared in every iteration.
 // TODO(verwaest): For-of has hacky position numbers for Symbol.iterator and
 // .next. Restore to proper positions once the CallPrinter can disambiguate
 // based on other values.
 var expected = [
   // Entry
-  "a2","b2",
+  "a2",
   // Empty for-in-var: get enumerable
   "c16",
   // Empty for-in: get enumerable
   "d12",
   // For-in-var: get enumerable, assign, body, assign, body, ...
   "e16","e11","E4","e11","E4","e11","E4","e11",
   // For-in: get enumerable, assign, body, assign, body, ...
   "f12","f7","F4","f7","F4","f7","F4","f7",
   // For-in-let: get enumerable, next, body, next,  ...
   "g16","g11","G4","g11","G4","g11","G4","g11",
@@ skipping 10 matching lines @@
   // For-let: init, condition, body, next, condition, body, ...
   "m7","m20","M4","m26","m20","M4","m26","m20","M4","m26","m20",
   // Exit.
   "y0","z0",
 ]
 print("expected:\n"+ JSON.stringify(expected));
 
 assertArrayEquals(expected, log);
 assertEquals(54, s);
 assertNull(exception);