Add types for the date-fns library (#11163)

date-fns/date-fns-tests.ts

@@ -0,0 +1,16 @@
+/// <reference path="date-fns.d.ts" />
+
+import {addDays, closestIndexTo, differenceInCalendarWeeks, max, isDate} from 'date-fns';
+import * as addHours from 'date-fns/add_hours';
+
+function test() {
+
+  addDays(new Date(), 5);
+  closestIndexTo(new Date(), [new Date(), new Date()]);
+  addHours(new Date(), 5);
+  differenceInCalendarWeeks(new Date(), new Date());
+  differenceInCalendarWeeks(new Date(), new Date(), {weekStartsOn: 1});
+  max(new Date(), new Date());
+  isDate({});
+
+}