Merge pull request #64 from holzgeist/fix-export-mediastreamtrack

fix: export mediastreamtrack

Author: cloudwebrtc

lib/dart_webrtc.dart

@@ -8,4 +8,5 @@ export 'src/factory_impl.dart';
 export 'src/media_devices.dart';
 export 'src/media_recorder.dart';
 export 'src/media_stream_impl.dart';
+export 'src/media_stream_track_impl.dart';
 export 'src/rtc_video_element.dart';