Workaround for canPlayType in Dartium

tests/html/audioelement_test.dart

 library AudioElementTest;
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
 import 'dart:html';
 
 main() {
   useHtmlConfiguration();
 
   test('constructorTest1', () {
       var audio = new AudioElement();
       expect(audio, isNotNull);
       expect(audio is AudioElement, isTrue);
     });
 
   test('constructorTest2', () {
       var audio = new AudioElement('IntentionallyMissingFileURL');
       expect(audio, isNotNull);
       expect(audio is AudioElement, isTrue);
       expect(audio.src, contains('IntentionallyMissingFileURL'));
     });
+
+  test('canPlayTypeTest', () {
+      var audio = new AudioElement();
+      var canPlay = audio.canPlayType("audio/mp4");
+      expect(canPlay, isNotNull);
+      expect(canPlay is String, isTrue);
+    });
 }