tests/html: removed usage of deprecated unittest features

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 23 matching lines @@
     context.fillStyle = 'red';
     context.fill();
 
     var url = canvas.toDataUrl();
 
     var img = new ImageElement();
     img.onLoad.listen(expectAsync((_) {
       expect(img.complete, true);
     }));
     img.onError.listen((_) {
-      guardAsync(() {
-        expect(true, isFalse, reason: 'URL failed to load.');
-      });
+      fail('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]));
   }
 }