[esnext] implement StringPad spec changes from March TC39 meeting

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