Remove the concept of document.documentElement

sky/tests/services/event-sender.sky

 #!mojo mojo:sky_viewer
 <sky>
 <script>
 import "../resources/third_party/unittest/unittest.dart";
 import "../resources/unit.dart";
 import "/sky/framework/shell.dart" as shell;
 import "dart:sky";
 import "package:mojo/services/input_events/public/interfaces/input_event_constants.mojom.dart" as constants;
 import "package:mojo/services/input_events/public/interfaces/input_events.mojom.dart" as events;
 import "package:sky/services/testing/test_harness.mojom.dart" as harness;
 
 void main() {
   initUnit();
 
   test('should be able to send events', () {
-    var sky = document.querySelector('sky');
-    sky.addEventListener('keypress', expectAsync((event) {
+    document.addEventListener('keypress', expectAsync((event) {
       expect(event.type, equals('keypress'));
       expect(event.key, equals(0));
       expect(event.charCode, equals(0x41));
     }));
 
     var testHarness = new harness.TestHarnessProxy.unbound();
     shell.connectToService("mojo:sky_tester", testHarness);
     var keyData = new events.KeyData();
     keyData.windowsKeyCode = 0x41;
     keyData.text = 0x41;
     keyData.unmodifiedText = 0x41;
     keyData.isChar = true;
     keyData.character = 0x41;
     var event = new events.Event();
     event.action = constants.EventType_KEY_PRESSED;
     event.keyData = keyData;
     testHarness.dispatchInputEvent(event);
   });
 }
 </script>
 </sky>