Make keySystem argument optional in MediaElement.canPlayType.

sdk/lib/html/dart2js/html_dart2js.dart

 /// The Dart HTML library.
 ///
 /// For examples, see
 /// [Dart HTML5 Samples](https://github.com/dart-lang/dart-html5-samples)
 /// on Github.
 library dart.dom.html;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:_collection-dev' hide Symbol;
@@ skipping 14372 matching lines @@
 
   @DomName('HTMLMediaElement.addTextTrack')
   @DocsEditable()
   // http://www.whatwg.org/specs/web-apps/current-work/multipage/the-video-element.html#dom-media-addtexttrack
   @Experimental()
   TextTrack addTextTrack(String kind, [String label, String language]) native;
 
   @DomName('HTMLMediaElement.canPlayType')
   @DocsEditable()
   @Unstable()
-  String canPlayType(String type, String keySystem) native;
+  String canPlayType(String type, [String keySystem]) native;
 
   @DomName('HTMLMediaElement.load')
   @DocsEditable()
   void load() native;
 
   @DomName('HTMLMediaElement.pause')
   @DocsEditable()
   void pause() native;
 
   @DomName('HTMLMediaElement.play')
@@ skipping 14919 matching lines @@
       _position = nextPosition;
       return true;
     }
     _current = null;
     _position = _array.length;
     return false;
   }
 
   T get current => _current;
 }