Converting tests over to using event streams.

tests/html/canvas_test.dart

Index: tests/html/canvas_test.dart
diff --git a/tests/html/canvas_test.dart b/tests/html/canvas_test.dart
index 79c48cb119462d113421e11e4a2493554af50380..6d95d3a2a2331e6cd4d59c808cca44d094f51a71 100644
--- a/tests/html/canvas_test.dart
+++ b/tests/html/canvas_test.dart
@@ -37,10 +37,10 @@ main() {
     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.');
       });