migrate semantics to nullsafety (#64055)

packages/flutter/analysis_options.yaml

@@ -7,5 +7,6 @@ analyzer:
     - non-nullable
   errors:
     always_require_non_null_named_parameters: false # not needed with nnbd
+    type_init_formals: false # https://github.com/dart-lang/linter/issues/2192
     void_checks: false # https://github.com/dart-lang/linter/issues/2185
     unnecessary_null_comparison: false # https://github.com/dart-lang/language/issues/1018 , turned off until https://github.com/flutter/flutter/issues/61042

packages/flutter/lib/semantics.dart

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 /// The Flutter semantics package.
 ///
 /// To use, import `package:flutter/semantics.dart`.

packages/flutter/lib/src/semantics/binding.dart

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
 
 import 'dart:ui' as ui show AccessibilityFeatures, SemanticsUpdateBuilder;
 
@@ -16,8 +15,8 @@ export 'dart:ui' show AccessibilityFeatures;
 // TODO(jonahwilliams): move the remaining semantic related bindings here.
 mixin SemanticsBinding on BindingBase {
   /// The current [SemanticsBinding], if one has been created.
-  static SemanticsBinding get instance => _instance;
-  static SemanticsBinding _instance;
+  static SemanticsBinding? get instance => _instance;
+  static SemanticsBinding? _instance;
 
   @override
   void initInstances() {
@@ -53,7 +52,7 @@ mixin SemanticsBinding on BindingBase {
   /// [WidgetsBindingObserver] and listen to
   /// [WidgetsBindingObserver.didChangeAccessibilityFeatures].
   ui.AccessibilityFeatures get accessibilityFeatures => _accessibilityFeatures;
-  ui.AccessibilityFeatures _accessibilityFeatures;
+  late ui.AccessibilityFeatures _accessibilityFeatures;
 
   /// The platform is requesting that animations be disabled or simplified.
   ///
@@ -63,7 +62,7 @@ mixin SemanticsBinding on BindingBase {
     bool value = _accessibilityFeatures.disableAnimations;
     assert(() {
       if (debugSemanticsDisableAnimations != null)
-        value = debugSemanticsDisableAnimations;
+        value = debugSemanticsDisableAnimations!;
       return true;
     }());
     return value;

packages/flutter/lib/src/semantics/debug.dart

@@ -2,10 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
 
 /// Overrides the setting of [SemanticsBinding.disableAnimations] for debugging
 /// and testing.
 ///
 /// This value is ignored in non-debug builds.
-bool debugSemanticsDisableAnimations;
+bool? debugSemanticsDisableAnimations;

packages/flutter/lib/src/semantics/semantics_event.dart

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
 
 import 'package:flutter/foundation.dart';
 import 'package:flutter/painting.dart';
@@ -30,7 +29,7 @@ abstract class SemanticsEvent {
   ///
   /// [nodeId] is the unique identifier of the semantics node associated with
   /// the event, or null if the event is not associated with a semantics node.
-  Map<String, dynamic> toMap({ int nodeId }) {
+  Map<String, dynamic> toMap({ int? nodeId }) {
     final Map<String, dynamic> event = <String, dynamic>{
       'type': type,
       'data': getDataMap(),

packages/flutter/lib/src/semantics/semantics_service.dart

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
 
 import 'dart:async';
 import 'dart:ui' show TextDirection;