perf(isArray): use native Array.isArray

see benchmark: http://jsperf.com/isarray-performance

Closes #7735

Author: sebholstein

src/Angular.js

@@ -510,10 +510,14 @@ function isDate(value) {
  * @param {*} value Reference to check.
  * @returns {boolean} True if `value` is an `Array`.
  */
-function isArray(value) {
-  return toString.call(value) === '[object Array]';
-}
-
+var isArray = (function() {
+  if (!isFunction(Array.isArray)) {
+    return function(value) {
+      return toString.call(value) === '[object Array]';
+    };
+  }
+  return Array.isArray;
+})();
 
 /**
  * @ngdoc function