Fix test_sky after 14270f1151aacfa650985c4333572c356d15c024

In order to make the Dart Analyzer stop complaining about
Event not having a default constructor (and all Event subclasses
not having an explicit constructor, thus having an implicit one
which calls the default Event() constructor), I made all of
Event()'s parameters named and thus optional, but did not update
our one sky test which used Event().

The real fix is probably to make the autogenerator smart enough
to have FooEvent() call super("foo"), but I was lazy.

This at least fixes the tests (and greens the waterfall) for now.

TBR=abarth@chromium.org
BUG=

Review URL: https://codereview.chromium.org/988573002

tests/events/custom-event.sky

@@ -5,7 +5,7 @@ import "../resources/unit.dart";
 import "dart:sky";
 
 class MyEvent extends Event {
-  MyEvent() : super("awesome", bubbles: true);
+  MyEvent() : super(type: "awesome", bubbles: true);
 
   bool get isCustom => true;
 }