[esnext] implement StringPad spec changes from March TC39 meeting

test/mjsunit/harmony/harmony-string-pad-end.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.
 
 // Flags: --harmony-string-padding
 
 (function TestMeta() {
   assertEquals(1, String.prototype.padEnd.length);
   assertEquals("function", typeof String.prototype.padEnd);
   assertEquals(Object.getPrototypeOf(Function),
@@ skipping 49 matching lines @@
   assertEquals("123", "123".padEnd(+0));
   assertEquals("123", "123".padEnd({ toString() { return +0; } }));
   assertEquals("123", "123".padEnd(NaN));
   assertEquals("123", "123".padEnd({ toString() { return NaN; } }));
 })();
 
 
 (function TestFillerToString() {
   assertEquals(".         ", ".".padEnd(10));
   assertEquals(".         ", ".".padEnd(10, undefined));
-  assertEquals(".         ", ".".padEnd(10, { toString() { return ""; } }));
   assertEquals(".nullnulln", ".".padEnd(10, null));
+  assertEquals(".XXXXXXXXX", ".".padEnd(10, { toString() { return "X"; } }));
+  assertEquals(
+      ".111111111",
+      ".".padEnd(10, { toString: undefined, valueOf() { return 1; } }));
 })();
 
 
+(function TestFillerEmptyString() {
+  assertEquals(".", ".".padEnd(10, ""));
+  assertEquals(".", ".".padEnd(10, { toString() { return ""; } }));
+  assertEquals(
+      ".", ".".padEnd(10, { toString: undefined, valueOf() { return ""; } }));
+})();
+
+
 (function TestFillerRepetition() {
   for (var i = 2000; i > 0; --i) {
     var expected = "123" + "xoxo".repeat(i / 4).slice(0, i - 3);
     var actual = "123".padEnd(i, "xoxo");
     assertEquals(expected, actual);
     assertEquals(i > "123".length ? i : 3, actual.length);
   }
 })();
 
 
 (function TestTruncation() {
   assertEquals("ab", "a".padEnd(2, "bc"));
 })();