Fix PointerAddedEvent handling in LiveTestWidgetsFlutterBinding (#61102)

* Fix PointerAddedEvent handling in tester

packages/flutter_test/lib/src/binding.dart

@@ -1433,8 +1433,11 @@ class LiveTestWidgetsFlutterBinding extends TestWidgetsFlutterBinding {
     switch (source) {
       case TestBindingEventSource.test:
         if (!renderView._pointers.containsKey(event.pointer)) {
-          assert(event.down);
-          renderView._pointers[event.pointer] = _LiveTestPointerRecord(event.pointer, event.position);
+          assert(event.down || event is PointerAddedEvent);
+          if (event.down) {
+            renderView._pointers[event.pointer] = _LiveTestPointerRecord(
+              event.pointer, event.position);
+          }
         } else {
           renderView._pointers[event.pointer].position = event.position;
           if (!event.down)

packages/flutter_test/test/live_binding_test.dart

+// Copyright 2014 The Flutter Authors. 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:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+// This file is for testings that require a `LiveTestWidgetsFlutterBinding`
+void main() {
+  LiveTestWidgetsFlutterBinding();
+  testWidgets('Input PointerAddedEvent', (WidgetTester tester) async {
+    await tester.pumpWidget(const MaterialApp(home: Text('Test')));
+    await tester.pump();
+    final TestGesture gesture = await tester.createGesture();
+    // This mimics the start of a gesture as seen on a device, where inputs
+    // starts with a PointerAddedEvent.
+    await gesture.addPointer();
+  });
+}