refactor(isArray): use Array.isArray exclusively

IE9 supports Array.isArray so we don't need a polyfill any more.

src/Angular.js

@@ -506,14 +506,7 @@ function isDate(value) {
  * @param {*} value Reference to check.
  * @returns {boolean} True if `value` is an `Array`.
  */
-var isArray = (function() {
-  if (!isFunction(Array.isArray)) {
-    return function(value) {
-      return toString.call(value) === '[object Array]';
-    };
-  }
-  return Array.isArray;
-})();
+var isArray = Array.isArray;
 
 /**
  * @ngdoc function