Remove unused dart:async imports. (#72410)

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

diff --git a/packages/flutter/lib/src/cupertino/refresh.dart b/packages/flutter/lib/src/cupertino/refresh.dart
index 7a33826..2d8653b 100644
--- a/packages/flutter/lib/src/cupertino/refresh.dart
+++ b/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

diff --git a/packages/flutter/lib/src/widgets/async.dart b/packages/flutter/lib/src/widgets/async.dart
index 8443bfb..6156b4f 100644
--- a/packages/flutter/lib/src/widgets/async.dart
+++ b/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';