Add MediaSource.supported, annotations for supported browsers, and a minimal test

sdk/lib/html/dart2js/html_dart2js.dart

Index: sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index 735b712df4db14ac6f9b68283678c5c578d89e9c..871d49e4ac5ee10a59775c45a9f800d7776ae264 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -19995,6 +19995,8 @@ class MediaQueryListEvent extends Event {
 
 @DocsEditable()
 @DomName('MediaSource')
+@SupportedBrowser(SupportedBrowser.CHROME)
+@SupportedBrowser(SupportedBrowser.IE, '11')
 // https://dvcs.w3.org/hg/html-media/raw-file/tip/media-source/media-source.html#mediasource
 @Experimental()
 @Native("MediaSource")
@@ -20009,6 +20011,9 @@ class MediaSource extends EventTarget {
   }
   static MediaSource _create_1() => JS('MediaSource', 'new MediaSource()');
 
+  /// Checks if this type is supported on the current platform.
+  static bool get supported => JS('bool', '!!(window.MediaSource)');
+
   @DomName('MediaSource.activeSourceBuffers')
   @DocsEditable()
   final SourceBufferList activeSourceBuffers;