Adding support for the MouseWheel event in Streams.

tests/html/wheelevent_test.dart

+library wheel_event_test;
+import '../../pkg/unittest/lib/unittest.dart';
+import '../../pkg/unittest/lib/html_config.dart';
+import 'dart:html';
+
+
+main() {
+
+  useHtmlConfiguration();
+
+  var userAgent = window.navigator.userAgent;
+
+  // Lame platform-dependent check to validate that our assumptions about
+  // which event is being used is correct.
+  var wheelEvent = 'wheel';
+  if (userAgent.contains("Opera", 0)) {
+    wheelEvent = 'mousewheel';
+  } else if (userAgent.contains("MSIE", 0)) {
+    wheelEvent = 'mousewheel';
+  } else if (userAgent.contains('Firefox')) {
+    // FF appears to have recently added support for wheel.
+    if (userAgent.contains('Firefox/17')) {
+      wheelEvent = 'DOMMouseScroll';
+    } else {
+      wheelEvent = 'wheel';
+    }
+  } else if (userAgent.contains('WebKit', 0)) {
+    wheelEvent = 'mousewheel';
+  }
+
+  test('wheelEvent', () {
+    var element = new DivElement();
+    element.on.mouseWheel.add(expectAsync1((e) {
+      expect(e.screenX, 100);
+      expect(e.deltaX, 0);
+      expect(e.deltaY, 240);
+    }));
+    var event = new WheelEvent(wheelEvent,
+      window,
+      0,
+      240,
+      0,
+      100,
+      200,
+      10,
+      20,
+      0);
+    element.$dom_dispatchEvent(event);
+  });
+
+  test('wheelEvent Stream', () {
+    var element = new DivElement();
+    element.onMouseWheel.listen(expectAsync1((e) {
+      expect(e.screenX, 100);
+      expect(e.deltaX, 0);
+      expect(e.deltaY, 240);
+    }));
+    var event = new WheelEvent(wheelEvent,
+      window,
+      0,
+      240,
+      0,
+      100,
+      200,
+      10,
+      20,
+      0);
+    element.$dom_dispatchEvent(event);
+  });
+}