perf: optimize internal isWindow call

Each window has a reference to itself, which is pretty unique so we can use
that to simplify our isWindow check

src/Angular.js

@@ -543,7 +543,7 @@ function isRegExp(value) {
  * @returns {boolean} True if `obj` is a window obj.
  */
 function isWindow(obj) {
-  return obj && obj.document && obj.location && obj.alert && obj.setInterval;
+  return obj && obj.window === obj;
 }