Update async_scope_capture_test to include asyncStart/asyncEnd.

Change-Id: Ibbf93373b594e2fc3e5909747d586c062376e6c0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/400760
Auto-Submit: Nate Biggs <[email protected]>
Reviewed-by: Nicholas Shahan <[email protected]>
Commit-Queue: Nate Biggs <[email protected]>

Author: natebiggs

tests/web/async_scope_capture_test.dart

@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+import 'package:expect/async_helper.dart';
+
 class Foo {
   bool disposed = false;
 
@@ -12,6 +14,7 @@ class Foo {
 }
 
 Future<void> main() async {
+  asyncStart();
   List<Function> callbacks = [];
   for (final x in [1, 2]) {
     final Foo foo = Foo();
@@ -24,4 +27,5 @@ Future<void> main() async {
   for (final callback in callbacks) {
     callback();
   }
+  asyncEnd();
 }