fix tests

packages/stream_chat_flutter/test/scrollable_positioned_list/horizontal_scrollable_positioned_list_test.dart

@@ -2,6 +2,7 @@
 // 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:ui';
 
 import 'package:flutter/material.dart';

packages/stream_chat_flutter/test/scrollable_positioned_list/reversed_scrollable_positioned_list_test.dart

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import 'dart:ui';
+import 'dart:async';
 
 import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';

packages/stream_chat_flutter/test/scrollable_positioned_list/scrollable_positioned_list_test.dart

@@ -2,6 +2,7 @@
 // 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 'dart:ui';
 

packages/stream_chat_flutter/test/scrollable_positioned_list/separated_scrollable_positioned_list_test.dart

@@ -2,6 +2,7 @@
 // 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:ui';
 
 import 'package:flutter/material.dart';

packages/stream_chat_flutter/test/scrollable_positioned_list/seperated_horizontal_scrollable_positioned_list_test.dart

@@ -2,6 +2,7 @@
 // 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:ui';
 
 import 'package:flutter/material.dart';