Support null safety about audio_session HOT 1 CLOSED

Package validation found the following potential issue:
* This package is opting into null-safety, but a dependency or file is not.
  
  package:sky_engine/js_util/js_util_dart2js.dart is opting out of null safety:
    ╷
  5 │ // @dart = 2.6
    │ ^^^^^^^^^^^^^^
    ╵
  
  Note that by publishing with non-migrated dependencies your package may be
  broken at any time if one of your dependencies migrates without a breaking 
  change release. 
  
  We highly recommend that you wait until all of your dependencies have been 
  migrated before publishing.
  
  Run `pub outdated --mode=null-safety` for more information about the state of
  dependencies.
  
  See https://dart.dev/null-safety/migration-guide
  for more information about migrating.