[esnext] implement StringPad spec changes from March TC39 meeting

src/js/harmony-string-padding.js

 // Copyright 2016 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 12 matching lines @@
   maxLength = TO_LENGTH(maxLength);
   var stringLength = thisString.length;
 
   if (maxLength <= stringLength) return "";
 
   if (IS_UNDEFINED(fillString)) {
     fillString = " ";
   } else {
     fillString = TO_STRING(fillString);
     if (fillString === "") {
-      fillString = " ";
+      // If filler is the empty String, return S.
+      return "";
     }
   }
 
   var fillLength = maxLength - stringLength;
   var repetitions = (fillLength / fillString.length) | 0;
   var remainingChars = (fillLength - fillString.length * repetitions) | 0;
 
   var filler = "";
   while (true) {
     if (repetitions & 1) filler += fillString;
@@ skipping 24 matching lines @@
   return thisString + StringPad(thisString, maxLength, fillString);
 }
 %FunctionSetLength(StringPadEnd, 1);
 
 utils.InstallFunctions(GlobalString.prototype, DONT_ENUM, [
   "padStart", StringPadStart,
   "padEnd", StringPadEnd
 ]);
 
 });