Workaround for canPlayType in Dartium

tests/html/audioelement_test.dart

Index: tests/html/audioelement_test.dart
diff --git a/tests/html/audioelement_test.dart b/tests/html/audioelement_test.dart
index 02c89359bb1ae9a9c78e2618888194978fa1b4fc..9957ff703e2fadacc2272e733b66be950d2f8d29 100644
--- a/tests/html/audioelement_test.dart
+++ b/tests/html/audioelement_test.dart
@@ -18,4 +18,11 @@ main() {
       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);
+    });
 }