[_fe_analyzer_shared] Remove unused dart:async import

Change-Id: I6f34f9c00399a6edd15ac50d17c11675d0e11f6e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/161465
Auto-Submit: Mike Fairhurst <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>
Reviewed-by: Konstantin Shcheglov <[email protected]>

Author: MichaelRFairhurst

pkg/_fe_analyzer_shared/lib/src/scanner/io.dart

@@ -4,8 +4,6 @@
 
 library _fe_analyzer_shared.scanner.io;
 
-import 'dart:async' show Future;
-
 import 'dart:io' show File, RandomAccessFile;
 
 import 'dart:typed_data' show Uint8List;