Fix nullability of GestureSettings on ViewConfig (flutter#27879)

lib/web_ui/lib/src/ui/platform_dispatcher.dart

@@ -190,7 +190,7 @@ class ViewConfiguration {
   final WindowPadding viewPadding;
   final WindowPadding systemGestureInsets;
   final WindowPadding padding;
-  final GestureSettings? gestureSettings;
+  final GestureSettings gestureSettings;
 
   @override
   String toString() {

lib/web_ui/test/gesture_settings_test.dart

@@ -0,0 +1,68 @@
+// Copyright 2013 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:test/bootstrap/browser.dart';
+import 'package:test/test.dart';
+import 'package:ui/ui.dart';
+
+void main() {
+  internalBootstrapBrowserTest(() => testMain);
+}
+
+void testMain() {
+  test('GestureSettings has a reasonable toString', () {
+    const GestureSettings gestureSettings =
+        GestureSettings(physicalDoubleTapSlop: 2.0, physicalTouchSlop: 1.0);
+
+    expect(gestureSettings.toString(),
+        'GestureSettings(physicalTouchSlop: 1, physicalDoubleTapSlop: 2)');
+  });
+
+  test('GestureSettings has a correct equality', () {
+    // don't refactor these to be const, that defeats the point!
+    final double value = nonconst(2.0);
+    final GestureSettings settingsA =
+        GestureSettings(physicalDoubleTapSlop: value, physicalTouchSlop: 1.0);
+    final GestureSettings settingsB =
+        GestureSettings(physicalDoubleTapSlop: value, physicalTouchSlop: 3.0);
+    final GestureSettings settingsC =
+        GestureSettings(physicalDoubleTapSlop: value, physicalTouchSlop: 1.0);
+
+    expect(settingsA, equals(settingsC));
+    expect(settingsC, equals(settingsA));
+
+    expect(settingsA, isNot(equals(settingsB)));
+    expect(settingsC, isNot(equals(settingsB)));
+
+    expect(settingsB, isNot(equals(settingsA)));
+    expect(settingsB, isNot(equals(settingsC)));
+  });
+
+  test('GestureSettings copyWith preserves already set values', () {
+    const GestureSettings initial =
+        GestureSettings(physicalDoubleTapSlop: 1.0, physicalTouchSlop: 1.0);
+
+    final GestureSettings copyA = initial.copyWith();
+
+    expect(copyA.physicalDoubleTapSlop, 1.0);
+    expect(copyA.physicalTouchSlop, 1.0);
+
+    final GestureSettings copyB =
+        copyA.copyWith(physicalDoubleTapSlop: 2.0, physicalTouchSlop: 2.0);
+
+    expect(copyB.physicalDoubleTapSlop, 2.0);
+    expect(copyB.physicalTouchSlop, 2.0);
+  });
+
+  test('GestureSettings constructor defaults to null', () {
+    const GestureSettings settings = GestureSettings();
+
+    expect(settings.physicalDoubleTapSlop, null);
+    expect(settings.physicalTouchSlop, null);
+  });
+}
+
+// Prevent the linter from complaining about a const value so that
+// non-identical equality can be tested.
+T nonconst<T>(T value) => value;