Set cancelable property on CustomEvent.

Commit e820d8a introduced a couple of
regressions:

1. Details not set properly. This was fixed by commit
288aa28.

2. It also failed to properly initialize the cancelable property. This
commit fixes it.

src/debuggability.js

@@ -152,7 +152,8 @@ var fireDomEvent = (function() {
             util.global.dispatchEvent(event);
             return function(name, event) {
                 var domEvent = new CustomEvent(name.toLowerCase(), {
-                    detail: event
+                    detail: event,
+                    cancelable: true
                 });
                 return !util.global.dispatchEvent(domEvent);
             };