migrate scheduler to nullsafety (#61570)

packages/flutter/lib/scheduler.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 Scheduler library.
 ///
 /// To use, import `package:flutter/scheduler.dart`.

packages/flutter/lib/src/scheduler/debug.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 'package:flutter/foundation.dart';
 
 // Any changes to this file should be reflected in the debugAssertAllSchedulerVarsUnset()

packages/flutter/lib/src/scheduler/priority.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 'package:flutter/foundation.dart';
 
 /// A task priority, as passed to [SchedulerBinding.scheduleTask].

packages/flutter/lib/src/scheduler/ticker.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 'package:flutter/foundation.dart';
@@ -71,7 +69,7 @@ class Ticker {
     }());
   }
 
-  TickerFuture _future;
+  TickerFuture? _future;
 
   /// Whether this ticker has been silenced.
   ///
@@ -115,9 +113,9 @@ class Ticker {
       return false;
     if (muted)
       return false;
-    if (SchedulerBinding.instance.framesEnabled)
+    if (SchedulerBinding.instance!.framesEnabled)
       return true;
-    if (SchedulerBinding.instance.schedulerPhase != SchedulerPhase.idle)
+    if (SchedulerBinding.instance!.schedulerPhase != SchedulerPhase.idle)
       return true; // for example, we might be in a warm-up frame or forced frame
     return false;
   }
@@ -130,7 +128,7 @@ class Ticker {
   /// [isTicking].
   bool get isActive => _future != null;
 
-  Duration _startTime;
+  Duration? _startTime;
 
   /// Starts the clock for this [Ticker]. If the ticker is not [muted], then this
   /// also starts calling the ticker's callback once per animation frame.
@@ -163,10 +161,10 @@ class Ticker {
     if (shouldScheduleTick) {
       scheduleTick();
     }
-    if (SchedulerBinding.instance.schedulerPhase.index > SchedulerPhase.idle.index &&
-        SchedulerBinding.instance.schedulerPhase.index < SchedulerPhase.postFrameCallbacks.index)
-      _startTime = SchedulerBinding.instance.currentFrameTimeStamp;
-    return _future;
+    if (SchedulerBinding.instance!.schedulerPhase.index > SchedulerPhase.idle.index &&
+        SchedulerBinding.instance!.schedulerPhase.index < SchedulerPhase.postFrameCallbacks.index)
+      _startTime = SchedulerBinding.instance!.currentFrameTimeStamp;
+    return _future!;
   }
 
   /// Adds a debug representation of a [Ticker] optimized for including in error
@@ -197,7 +195,7 @@ class Ticker {
     // We take the _future into a local variable so that isTicking is false
     // when we actually complete the future (isTicking uses _future to
     // determine its state).
-    final TickerFuture localFuture = _future;
+    final TickerFuture localFuture = _future!;
     _future = null;
     _startTime = null;
     assert(!isActive);
@@ -213,7 +211,7 @@ class Ticker {
 
   final TickerCallback _onTick;
 
-  int _animationId;
+  int? _animationId;
 
   /// Whether this [Ticker] has already scheduled a frame callback.
   @protected
@@ -237,7 +235,7 @@ class Ticker {
     _animationId = null;
 
     _startTime ??= timeStamp;
-    _onTick(timeStamp - _startTime);
+    _onTick(timeStamp - _startTime!);
 
     // The onTick callback may have scheduled another tick already, for
     // example by calling stop then start again.
@@ -252,7 +250,7 @@ class Ticker {
   void scheduleTick({ bool rescheduling = false }) {
     assert(!scheduled);
     assert(shouldScheduleTick);
-    _animationId = SchedulerBinding.instance.scheduleFrameCallback(_tick, rescheduling: rescheduling);
+    _animationId = SchedulerBinding.instance!.scheduleFrameCallback(_tick, rescheduling: rescheduling);
   }
 
   /// Cancels the frame callback that was requested by [scheduleTick], if any.
@@ -264,7 +262,7 @@ class Ticker {
   @protected
   void unscheduleTick() {
     if (scheduled) {
-      SchedulerBinding.instance.cancelFrameCallbackWithId(_animationId);
+      SchedulerBinding.instance!.cancelFrameCallbackWithId(_animationId!);
       _animationId = null;
     }
     assert(!shouldScheduleTick);
@@ -301,7 +299,7 @@ class Ticker {
   @mustCallSuper
   void dispose() {
     if (_future != null) {
-      final TickerFuture localFuture = _future;
+      final TickerFuture localFuture = _future!;
       _future = null;
       assert(!isActive);
       unscheduleTick();
@@ -319,8 +317,8 @@ class Ticker {
   /// An optional label can be provided for debugging purposes.
   ///
   /// This label will appear in the [toString] output in debug builds.
-  final String debugLabel;
-  StackTrace _debugCreationStack;
+  final String? debugLabel;
+  late StackTrace _debugCreationStack;
 
   @override
   String toString({ bool debugIncludeStack = false }) {
@@ -374,14 +372,14 @@ class TickerFuture implements Future<void> {
   }
 
   final Completer<void> _primaryCompleter = Completer<void>();
-  Completer<void> _secondaryCompleter;
-  bool _completed; // null means unresolved, true means complete, false means canceled
+  Completer<void>? _secondaryCompleter;
+  bool? _completed; // null means unresolved, true means complete, false means canceled
 
   void _complete() {
     assert(_completed == null);
     _completed = true;
-    _primaryCompleter.complete(null);
-    _secondaryCompleter?.complete(null);
+    _primaryCompleter.complete();
+    _secondaryCompleter?.complete();
   }
 
   void _cancel(Ticker ticker) {
@@ -415,14 +413,14 @@ class TickerFuture implements Future<void> {
     if (_secondaryCompleter == null) {
       _secondaryCompleter = Completer<void>();
       if (_completed != null) {
-        if (_completed) {
-          _secondaryCompleter.complete();
+        if (_completed!) {
+          _secondaryCompleter!.complete();
         } else {
-          _secondaryCompleter.completeError(const TickerCanceled());
+          _secondaryCompleter!.completeError(const TickerCanceled());
         }
       }
     }
-    return _secondaryCompleter.future;
+    return _secondaryCompleter!.future;
   }
 
   @override
@@ -431,17 +429,17 @@ class TickerFuture implements Future<void> {
   }
 
   @override
-  Future<void> catchError(Function onError, { bool test(dynamic error) }) {
+  Future<void> catchError(Function onError, { bool Function(Object)? test }) {
     return _primaryCompleter.future.catchError(onError, test: test);
   }
 
   @override
-  Future<R> then<R>(FutureOr<R> onValue(void value), { Function onError }) {
+  Future<R> then<R>(FutureOr<R> onValue(void value), { Function? onError }) {
     return _primaryCompleter.future.then<R>(onValue, onError: onError);
   }
 
   @override
-  Future<void> timeout(Duration timeLimit, { dynamic onTimeout() }) {
+  Future<void> timeout(Duration timeLimit, { FutureOr<void> Function()? onTimeout }) {
     return _primaryCompleter.future.timeout(timeLimit, onTimeout: onTimeout);
   }
 
@@ -451,7 +449,7 @@ class TickerFuture implements Future<void> {
   }
 
   @override
-  String toString() => '${describeIdentity(this)}(${ _completed == null ? "active" : _completed ? "complete" : "canceled" })';
+  String toString() => '${describeIdentity(this)}(${ _completed == null ? "active" : _completed! ? "complete" : "canceled" })';
 }
 
 /// Exception thrown by [Ticker] objects on the [TickerFuture.orCancel] future
@@ -464,7 +462,7 @@ class TickerCanceled implements Exception {
   ///
   /// This may be null in the case that the [Future] created for
   /// [TickerFuture.orCancel] was created after the ticker was canceled.
-  final Ticker ticker;
+  final Ticker? ticker;
 
   @override
   String toString() {