Add a comment explaining why a binding set to window.Event needs an explicit type.

Author: bicknellr

packages/formdata-event/ts_src/environment/event.ts

@@ -9,6 +9,9 @@
  * additional IP rights grant found at http://polymer.github.io/PATENTS.txt
  */
 
+// Without this explicit type, TS seems to think that `window.Event` is the
+// wrapper from `wrappers/event.ts` at this point, which is typed in terms of
+// this binding, causing a cycle in its type.
 export const constructor: typeof Event = window.Event;
 
 export const prototype = constructor.prototype;