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

sdk/lib/html/dartium/html_dartium.dart

Index: sdk/lib/html/dartium/html_dartium.dart
diff --git a/sdk/lib/html/dartium/html_dartium.dart b/sdk/lib/html/dartium/html_dartium.dart
index 2a991bd2c065d982ee62fad3d970e70237135933..84bf5fd535f7ef7fe5a36ac55c992fd295db7984 100644
--- a/sdk/lib/html/dartium/html_dartium.dart
+++ b/sdk/lib/html/dartium/html_dartium.dart
@@ -22046,6 +22046,8 @@ 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()
 class MediaSource extends EventTarget {
@@ -22058,6 +22060,9 @@ class MediaSource extends EventTarget {
     return _blink.BlinkMediaSource.instance.constructorCallback_0_();
   }
 
+  /// Checks if this type is supported on the current platform.
+  static bool get supported => true;
+
   @DomName('MediaSource.activeSourceBuffers')
   @DocsEditable()
   SourceBufferList get activeSourceBuffers => _blink.BlinkMediaSource.instance.activeSourceBuffers_Getter_(this);