fix #19175 How should addTime be used from a test? (#25202)

packages/flutter_test/lib/src/binding.dart

@@ -125,6 +125,11 @@ abstract class TestWidgetsFlutterBinding extends BindingBase
   @protected
   bool get disableShadows => false;
 
+  /// Increase the timeout for the current test by the given duration.
+  void addTime(Duration duration) {
+    // Noop, see [AutomatedTestWidgetsFlutterBinding. addTime] for an actual implementation.
+  }
+
   /// The value to set [debugCheckIntrinsicSizes] to while tests are running.
   ///
   /// This can be used to enable additional checks. For example,
@@ -843,7 +848,7 @@ class AutomatedTestWidgetsFlutterBinding extends TestWidgetsFlutterBinding {
       _timeoutCompleter.completeError(
         TimeoutException(
           'The test exceeded the timeout. It may have hung.\n'
-          'Consider using "addTime" to increase the timeout before expensive operations.',
+          'Consider using "tester.binding.addTime" to increase the timeout before expensive operations.',
           _timeout,
         ),
       );
@@ -873,6 +878,7 @@ class AutomatedTestWidgetsFlutterBinding extends TestWidgetsFlutterBinding {
   ///
   ///  * [defaultTestTimeout], the maximum that the timeout can reach.
   ///    (That timeout is implemented by the test package.)
+  @override
   void addTime(Duration duration) {
     assert(_timeout != null, 'addTime can only be called during a test.');
     _timeout += duration;

packages/flutter_test/lib/src/widget_tester.dart

@@ -49,6 +49,14 @@ typedef WidgetTesterCallback = Future<void> Function(WidgetTester widgetTester);
 ///
 /// The callback can be asynchronous (using `async`/`await` or
 /// using explicit [Future]s).
+/// Tests using the [AutomatedTestWidgetsFlutterBinding]
+/// have a default time out of two seconds,
+/// which is automatically increased for some expensive operations,
+/// and can also be manually increased by calling
+/// [AutomatedTestWidgetsFlutterBinding.addTime].
+/// The maximum that this timeout can reach (automatically or manually increased)
+/// is defined by the timeout property,
+/// which defaults to [TestWidgetsFlutterBinding.defaultTestTimeout].
 ///
 /// This function uses the [test] function in the test package to
 /// register the given callback as a test. The callback, when run,
@@ -56,6 +64,11 @@ typedef WidgetTesterCallback = Future<void> Function(WidgetTester widgetTester);
 /// provides convenient widget [Finder]s for use with the
 /// [WidgetTester].
 ///
+/// See also:
+///
+///  * [AutomatedTestWidgetsFlutterBinding.addTime] to learn more about
+/// timeout and how to manually increase timeouts.
+///
 /// ## Sample code
 ///
 /// ```dart