Split off a separate --harmony_sloppy_let flag

test/mjsunit/harmony/block-scoping-top-level-sloppy.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: --min-preparse-length=0
-// Flags: --no-legacy-const --harmony-sloppy
+// Flags: --no-legacy-const --harmony-sloppy --harmony-sloppy-let
 
 let xxx = 1;
 let f = undefined;
 {
   let inner_x = xxx;
   f = function() { return inner_x; };
 }
 
 assertSame(1, f());
 
 xxx = 42;
 {
   f = function() { return inner_x1; };
   let inner_x1 = xxx;
 }
 
 assertSame(42, f());
 
 xxx = 31;
 {
   let inner_x1 = xxx;
   try {
     throw new Error();
   } catch (e) {
     f = function() { return inner_x1; };
   }
 }
 assertSame(31, f());