Fix keyed stores to strings convertible to indices

test/mjsunit/primitive-keyed-access.js

 // Copyright 2015 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.
 
 Object.defineProperty(Number.prototype, "0",
     { set: function(v) { set = v; }});
 Object.defineProperty(String.prototype, "0",
     { set: function(v) { set = v; }});
 Object.defineProperty(String.prototype, "3",
     { set: function(v) { set = v; }});
@@ skipping 23 matching lines @@
   "use strict";
   var sym = Symbol('66');
   sym['a' + 'b'] = 0;
 });
 
 assertThrows(function() {
   "use strict";
   var sym = Symbol('66');
   sym[62] = 0;
 });
+
+assertThrows(function() {
+  "use strict";
+  var o = "bla";
+  o["0"] = 1;
+});