[es6] Use SubString in String{Starts,Ends}With

src/string.js

 // Copyright 2012 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.
 
 (function(global, utils) {
 
 %CheckIsBootstrapping();
 
 // -------------------------------------------------------------------
 // Imports
@@ skipping 979 matching lines @@
     pos = $toInteger(pos);
   }
 
   var s_len = s.length;
   var start = MathMin(MathMax(pos, 0), s_len);
   var ss_len = ss.length;
   if (ss_len + start > s_len) {
     return false;
   }
 
-  return %StringIndexOf(s, ss, start) === start;
+  return %_SubString(s, start, start + ss_len) === ss;
 }
 
 
 // ES6 draft 04-05-14, section 21.1.3.7
 function StringEndsWith(searchString /* position */) {  // length == 1
   CHECK_OBJECT_COERCIBLE(this, "String.prototype.endsWith");
 
   var s = TO_STRING_INLINE(this);
 
   if (IS_REGEXP(searchString)) {
@@ skipping 10 matching lines @@
     }
   }
 
   var end = MathMin(MathMax(pos, 0), s_len);
   var ss_len = ss.length;
   var start = end - ss_len;
   if (start < 0) {
     return false;
   }
 
-  return %StringLastIndexOf(s, ss, start) === start;
+  return %_SubString(s, start, start + ss_len) === ss;
 }
 
 
 // ES6 draft 04-05-14, section 21.1.3.6
 function StringIncludes(searchString /* position */) {  // length == 1
   CHECK_OBJECT_COERCIBLE(this, "String.prototype.includes");
 
   var string = TO_STRING_INLINE(this);
 
   if (IS_REGEXP(searchString)) {
@@ skipping 164 matching lines @@
   to.StringLastIndexOf = StringLastIndexOfJS;
   to.StringMatch = StringMatchJS;
   to.StringReplace = StringReplace;
   to.StringSlice = StringSlice;
   to.StringSplit = StringSplitJS;
   to.StringSubstr = StringSubstr;
   to.StringSubstring = StringSubstring;
 });
 
 })