Comments around hanging tests via runAsync. (#60136)

packages/flutter_test/lib/src/binding.dart

@@ -362,7 +362,7 @@ abstract class TestWidgetsFlutterBinding extends BindingBase
   ///
   /// If `callback` completes with an error, the error will be caught by the
   /// Flutter framework and made available via [takeException], and this method
-  /// will return a future that completes will `null`.
+  /// will return a future that completes with `null`.
   ///
   /// Re-entrant calls to this method are not allowed; callers of this method
   /// are required to wait for the returned future to complete before calling

packages/flutter_test/lib/src/widget_tester.dart

@@ -603,12 +603,24 @@ class WidgetTester extends WidgetController implements HitTestDispatcher, Ticker
   ///
   /// If [callback] completes with an error, the error will be caught by the
   /// Flutter framework and made available via [takeException], and this method
-  /// will return a future that completes will `null`.
+  /// will return a future that completes with `null`.
   ///
   /// Re-entrant calls to this method are not allowed; callers of this method
   /// are required to wait for the returned future to complete before calling
   /// this method again. Attempts to do otherwise will result in a
   /// [TestFailure] error being thrown.
+  ///
+  /// If your widget test hangs and you are using [runAsync], chances are your
+  /// code depends on the result of a task that did not complete. Fake async
+  /// environment is unable to resolve a future that was created in [runAsync].
+  /// If you observe such behavior or flakiness, you have a number of options:
+  ///
+  /// * Consider restructuring your code so you do not need [runAsync]. This is
+  ///   the optimal solution as widget tests are designed to run in fake async
+  ///   environment.
+  ///
+  /// * Expose a [Future] in your application code that signals the readiness of
+  ///   your widget tree, then await that future inside [callback].
   Future<T> runAsync<T>(
     Future<T> callback(), {
     Duration additionalTime = const Duration(milliseconds: 1000),