[es6] Allow any valid repeat of empty string in String.prototype.repeat

src/js/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 825 matching lines @@
   return "<sub>" + TO_STRING(this) + "</sub>";
 }
 
 
 // ES6 draft, revision 26 (2014-07-18), section B.2.3.14
 function StringSup() {
   CHECK_OBJECT_COERCIBLE(this, "String.prototype.sup");
   return "<sup>" + TO_STRING(this) + "</sup>";
 }
 
-// ES6 draft 01-20-14, section 21.1.3.13
+// ES6, section 21.1.3.13
 function StringRepeat(count) {
   CHECK_OBJECT_COERCIBLE(this, "String.prototype.repeat");
 
   var s = TO_STRING(this);
   var n = TO_INTEGER(count);
+
+  if (n < 0 || n === INFINITY) throw MakeRangeError(kInvalidCountValue);
+
+  // Early return to allow an arbitrarily-large repeat of the empty string.
+  if (s.length === 0) return "";
+
   // The maximum string length is stored in a smi, so a longer repeat
   // must result in a range error.
-  if (n < 0 || n > %_MaxSmi()) throw MakeRangeError(kInvalidCountValue);
+  if (n > %_MaxSmi()) throw MakeRangeError(kInvalidCountValue);
 
   var r = "";
   while (true) {
     if (n & 1) r += s;
     n >>= 1;
     if (n === 0) return r;
     s += s;
   }
 }
 
@@ skipping 232 matching lines @@
   to.StringLastIndexOf = StringLastIndexOfJS;
   to.StringMatch = StringMatchJS;
   to.StringReplace = StringReplace;
   to.StringSlice = StringSlice;
   to.StringSplit = StringSplitJS;
   to.StringSubstr = StringSubstr;
   to.StringSubstring = StringSubstring;
 });
 
 })