Remove unused dart:async imports. (#72410)

packages/flutter/lib/src/cupertino/refresh.dart

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'dart:async';
 import 'dart:math';
 
 import 'package:flutter/rendering.dart';

packages/flutter/lib/src/widgets/async.dart

@@ -6,7 +6,7 @@
 ///
 /// Asynchronous computations are represented by [Future]s and [Stream]s.
 
-import 'dart:async' show Future, Stream, StreamSubscription;
+import 'dart:async' show StreamSubscription;
 
 import 'package:flutter/foundation.dart';