Fix error in es6symbol with IE8 (#2630)

Keyword 'for' in `Symbol.for` caused `Undefined identifier` error in IE8.

feature-detects/es6/symbol.js

@@ -20,7 +20,7 @@ Check if browser implements ECMAScript 6 Symbol per specification.
 */
 define(['Modernizr'], function(Modernizr) {
   Modernizr.addTest('es6symbol', !!(typeof(Symbol)==="function" &&
-    Symbol.for &&
+    Symbol['for'] &&
     Symbol.hasInstance &&
     Symbol.isConcatSpreadable &&
     Symbol.iterator &&