Keep track of the number of events we have registered.

This is needed to reliably prevent the handleBrowserEvent method to perform potentially expensive calculations.

test/spec/ol/Events.test.js

@@ -114,6 +114,8 @@ describe("ol.event.Events", function() {
         
         log = [];
         events.register("bar", logFn);
+        expect(events.listenerCount_["bar"]).toBe(1);
+
         events.triggerEvent("bar");
         expect(log.length).toBe(1);
         
@@ -121,6 +123,9 @@ describe("ol.event.Events", function() {
         events.triggerEvent("bar");
         expect(log.length).toBe(1);
         
+        events.unregister("bar", logFn);
+        expect(events.listenerCount_["bar"]).toBe(0);
+        
         events.destroy();
     });