Update part directive in broadcast_channel.dart

Author: terrier989

lib/src/html/api/broadcast_channel.dart

@@ -1,4 +1,4 @@
-part of universal_html.internal;
+part of '../../html.dart';
 
 class BroadcastChannel {
   final String name;
@@ -10,4 +10,4 @@ class BroadcastChannel {
   void postMessage(Object message) {}
 
   Stream<dynamic> get onMessage => const Stream.empty();
-}
+}