Converting tests over to using event streams.

tests/html/canvas_test.dart

 library CanvasTest;
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_config.dart';
 import 'dart:html';
 
 main() {
   CanvasElement canvas;
   CanvasRenderingContext2D context;
   int width = 100;
   int height = 100;
@@ skipping 19 matching lines @@
 
   test('toDataUrl', () {
     var canvas = new CanvasElement(width: 100, height: 100);
     var context = canvas.context2d;
     context.fillStyle = 'red';
     context.fill();
 
     var url = canvas.toDataUrl('image/png');
 
     var img = new ImageElement();
-    img.on.load.add(expectAsync1((_) {
+    img.onLoad.listen(expectAsync1((_) {
       expect(img.complete, true);
     }));
-    img.on.error.add((_) {
+    img.onError.listen((_) {
       guardAsync(() {
         expect(true, isFalse, reason: 'URL failed to load.');
       });
     });
     img.src = url;
   });
 }
 
 void checkPixel(List<int> data, int offset, List<int> rgba)
 {
   offset *= 4;
   for (var i = 0; i < 4; ++i) {
     expect(data[offset + i], equals(rgba[i]));
   }
 }