Make it possible to create reasonable subclasses of Event

sky/tests/events/custom-event.sky

+<script>
+import "../resources/third_party/unittest/unittest.dart";
+import "../resources/unit.dart";
+
+import "dart:sky";
+
+class MyEvent extends Event {
+  MyEvent() : super("awesome", bubbles: true);
+
+  bool get isCustom => true;
+}
+
+void main() {
+  initUnit();
+
+  test("should be able to dispatch", () {
+    var event = new MyEvent();
+    expect(event.isCustom, isTrue);
+    expect(event.type, equals("awesome"));
+    expect(event.bubbles, isTrue);
+    expect(event.cancelable, isFalse);
+    expect(event.isCustom, isTrue);
+
+    bool gotEvent = false;
+    var element = document.createElement("div");
+    element.addEventListener("awesome", (e) {
+      expect(e, equals(event));
+      expect(event.isCustom, isTrue);
+      gotEvent = true;
+    });
+    element.dispatchEvent(event);
+    expect(gotEvent, isTrue);
+  });
+}
+</script>