Export `elapseBlocking` to test binding, so slow sync work can be simulated...

Export `elapseBlocking` to test binding, so slow sync work can be simulated such as a slow widget build (#112619)

packages/flutter_test/lib/src/binding.dart

@@ -1277,6 +1277,12 @@ class AutomatedTestWidgetsFlutterBinding extends TestWidgetsFlutterBinding {
     return Future<void>.value();
   }
 
+  /// Simulates the synchronous passage of time, resulting from blocking or
+  /// expensive calls.
+  void elapseBlocking(Duration duration) {
+    _currentFakeAsync!.elapseBlocking(duration);
+  }
+
   @override
   Future<void> runTest(
     Future<void> Function() testBody,

packages/flutter_test/test/bindings_test.dart

@@ -7,7 +7,7 @@
 // https://github.com/flutter/flutter/issues/85160
 // Fails with "flutter test --test-randomize-ordering-seed=20210721"
 @Tags(<String>['no-shuffle'])
-
+import 'dart:async';
 import 'dart:io';
 
 import 'package:flutter/widgets.dart';
@@ -56,4 +56,38 @@ void main() {
     expect(binding.testTextInput.isRegistered, isFalse);
     order += 1;
   });
+
+  group('elapseBlocking', () {
+    testWidgets('timer is not called', (WidgetTester tester) async {
+      bool timerCalled = false;
+      Timer.run(() => timerCalled = true);
+
+      binding.elapseBlocking(const Duration(seconds: 1));
+
+      expect(timerCalled, false);
+      binding.idle();
+    });
+
+    testWidgets('can use to simulate slow build', (WidgetTester tester) async {
+      final DateTime beforeTime = binding.clock.now();
+
+      await tester.pumpWidget(Builder(builder: (_) {
+        bool timerCalled = false;
+        Timer.run(() => timerCalled = true);
+
+        binding.elapseBlocking(const Duration(seconds: 1));
+
+        // if we use `delayed` instead of `elapseBlocking`, such as
+        // binding.delayed(const Duration(seconds: 1));
+        // the timer will be called here. Surely, that violates how
+        // a flutter widget build works
+        expect(timerCalled, false);
+
+        return Container();
+      }));
+
+      expect(binding.clock.now(), beforeTime.add(const Duration(seconds: 1)));
+      binding.idle();
+    });
+  });
 }