TF-2460 Use conditional export for BroadcastChannel to avoid failed compilation

core/lib/utils/broadcast_channel/broadcast_channel.dart

@@ -0,0 +1 @@
+export 'broadcast_channel_stub.dart' if (dart.library.html) 'broadcast_channel_web.dart';

core/lib/utils/broadcast_channel/broadcast_channel_stub.dart

@@ -0,0 +1,7 @@
+class BroadcastChannel {
+  final String name;
+
+  BroadcastChannel(this.name);
+
+  Stream<dynamic> get onMessage => const Stream.empty();
+}

core/lib/utils/broadcast_channel/broadcast_channel_web.dart

@@ -0,0 +1 @@
+export 'dart:html' show BroadcastChannel;