[flutter_tools] migrate context to null safety (#78359)

packages/flutter_tools/lib/src/base/context.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
-
 import 'dart:async';
 import 'dart:collection';
 
@@ -39,7 +37,7 @@ const Object contextKey = _Key.key;
 /// context will not have any values associated with it.
 ///
 /// This is guaranteed to never return `null`.
-AppContext get context => Zone.current[contextKey] as AppContext ?? AppContext._root;
+AppContext get context => Zone.current[contextKey] as AppContext? ?? AppContext._root;
 
 /// A lookup table (mapping types to values) and an implied scope, in which
 /// code is run.
@@ -58,13 +56,13 @@ class AppContext {
     this._fallbacks = const <Type, Generator>{},
   ]);
 
-  final String name;
-  final AppContext _parent;
+  final String? name;
+  final AppContext? _parent;
   final Map<Type, Generator> _overrides;
   final Map<Type, Generator> _fallbacks;
   final Map<Type, dynamic> _values = <Type, dynamic>{};
 
-  List<Type> _reentrantChecks;
+  List<Type>? _reentrantChecks;
 
   /// Bootstrap context.
   static final AppContext _root = AppContext._(null, 'ROOT');
@@ -94,19 +92,19 @@ class AppContext {
     return _values.putIfAbsent(type, () {
       _reentrantChecks ??= <Type>[];
 
-      final int index = _reentrantChecks.indexOf(type);
+      final int index = _reentrantChecks!.indexOf(type);
       if (index >= 0) {
         // We're already in the process of trying to generate this type.
         throw ContextDependencyCycleException._(
-            UnmodifiableListView<Type>(_reentrantChecks.sublist(index)));
+            UnmodifiableListView<Type>(_reentrantChecks!.sublist(index)));
       }
 
-      _reentrantChecks.add(type);
+      _reentrantChecks!.add(type);
       try {
-        return _boxNull(generators[type]());
+        return _boxNull(generators[type]!());
       } finally {
-        _reentrantChecks.removeLast();
-        if (_reentrantChecks.isEmpty) {
+        _reentrantChecks!.removeLast();
+        if (_reentrantChecks!.isEmpty) {
           _reentrantChecks = null;
         }
       }
@@ -118,7 +116,7 @@ class AppContext {
   T get<T>() {
     dynamic value = _generateIfNecessary(T, _overrides);
     if (value == null && _parent != null) {
-      value = _parent.get<T>();
+      value = _parent!.get<T>();
     }
     return _unboxNull(value ?? _generateIfNecessary(T, _fallbacks)) as T;
   }
@@ -136,11 +134,11 @@ class AppContext {
   /// name. This is useful for debugging purposes and is analogous to naming a
   /// thread in Java.
   Future<V> run<V>({
-    @required FutureOr<V> Function() body,
-    String name,
-    Map<Type, Generator> overrides,
-    Map<Type, Generator> fallbacks,
-    ZoneSpecification zoneSpecification,
+    required FutureOr<V> Function() body,
+    String? name,
+    Map<Type, Generator>? overrides,
+    Map<Type, Generator>? fallbacks,
+    ZoneSpecification? zoneSpecification,
   }) async {
     final AppContext child = AppContext._(
       this,
@@ -159,7 +157,7 @@ class AppContext {
   String toString() {
     final StringBuffer buf = StringBuffer();
     String indent = '';
-    AppContext ctx = this;
+    AppContext? ctx = this;
     while (ctx != null) {
       buf.write('AppContext');
       if (ctx.name != null) {