migrate rendering to nullsafety (#64621)

analysis_options.yaml

@@ -202,6 +202,7 @@ linter:
     - use_full_hex_values_for_flutter_colors
     # - use_function_type_syntax_for_parameters # not yet tested
     # - use_key_in_widget_constructors # not yet tested
+    - use_late_for_private_fields_and_variables
     - use_rethrow_when_possible
     # - use_setters_to_change_properties # not yet tested
     # - use_string_buffers # has false positives: https://github.com/dart-lang/sdk/issues/34182

packages/flutter/analysis_options.yaml

@@ -8,5 +8,6 @@ analyzer:
   errors:
     always_require_non_null_named_parameters: false # not needed with nnbd
     type_init_formals: false # https://github.com/dart-lang/linter/issues/2192
+    unrelated_type_equality_checks: false # https://github.com/dart-lang/linter/issues/2196
     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/rendering.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 rendering tree.
 ///
 /// To use, import `package:flutter/rendering.dart`.

packages/flutter/lib/src/animation/tween.dart

@@ -208,6 +208,20 @@ class _ChainedEvaluation<T> extends Animatable<T> {
 /// [Tween]s that use dedicated `lerp` methods instead of merely relying on the
 /// operators (in particular, this allows them to handle null values in a more
 /// useful manner).
+///
+/// ## Nullability
+///
+/// The [begin] and [end] fields are nullable; a [Tween] does not have to
+/// have non-null values specified when it is created.
+///
+/// If `T` is nullable, then [lerp] and [transform] may return null.
+/// This is typically seen in the case where [begin] is null and `t`
+/// is 0.0, or [end] is null and `t` is 1.0, or both are null (at any
+/// `t` value).
+///
+/// If `T` is not nullable, then [begin] and [end] must both be set to
+/// non-null values before using [lerp] or [transform], otherwise they
+/// will throw.
 class Tween<T extends dynamic> extends Animatable<T> {
   /// Creates a tween.
   ///
@@ -236,6 +250,9 @@ class Tween<T extends dynamic> extends Animatable<T> {
   /// The default implementation of this method uses the [+], [-], and [*]
   /// operators on `T`. The [begin] and [end] properties must therefore be
   /// non-null by the time this method is called.
+  ///
+  /// In general, however, it is possible for this to return null, especially
+  /// when `t`=0.0 and [begin] is null, or `t`=1.0 and [end] is null.
   @protected
   T lerp(double t) {
     assert(begin != null);
@@ -291,6 +308,9 @@ class ReverseTween<T> extends Tween<T> {
 /// This class specializes the interpolation of [Tween<Color>] to use
 /// [Color.lerp].
 ///
+/// The values can be null, representing no color (which is distinct to
+/// transparent black, as represented by [Colors.transparent]).
+///
 /// See [Tween] for a discussion on how to use interpolation objects.
 class ColorTween extends Tween<Color?> {
   /// Creates a [Color] tween.
@@ -314,6 +334,8 @@ class ColorTween extends Tween<Color?> {
 /// This class specializes the interpolation of [Tween<Size>] to use
 /// [Size.lerp].
 ///
+/// The values can be null, representing [Size.zero].
+///
 /// See [Tween] for a discussion on how to use interpolation objects.
 class SizeTween extends Tween<Size?> {
   /// Creates a [Size] tween.
@@ -332,6 +354,9 @@ class SizeTween extends Tween<Size?> {
 /// This class specializes the interpolation of [Tween<Rect>] to use
 /// [Rect.lerp].
 ///
+/// The values can be null, representing a zero-sized rectangle at the
+/// origin ([Rect.zero]).
+///
 /// See [Tween] for a discussion on how to use interpolation objects.
 class RectTween extends Tween<Rect?> {
   /// Creates a [Rect] tween.
@@ -355,6 +380,9 @@ class RectTween extends Tween<Rect?> {
 /// This is the closest approximation to a linear tween that is possible with an
 /// integer. Compare to [StepTween] and [Tween<double>].
 ///
+/// The [begin] and [end] values must be set to non-null values before
+/// calling [lerp] or [transform].
+///
 /// See [Tween] for a discussion on how to use interpolation objects.
 class IntTween extends Tween<int> {
   /// Creates an int tween.
@@ -380,6 +408,9 @@ class IntTween extends Tween<int> {
 /// This results in a value that is never greater than the equivalent
 /// value from a linear double interpolation. Compare to [IntTween].
 ///
+/// The [begin] and [end] values must be set to non-null values before
+/// calling [lerp] or [transform].
+///
 /// See [Tween] for a discussion on how to use interpolation objects.
 class StepTween extends Tween<int> {
   /// Creates an [int] tween that floors.

packages/flutter/lib/src/foundation/diagnostics.dart

@@ -3377,13 +3377,10 @@ abstract class DiagnosticableTree with Diagnosticable {
   ///
   ///  * [toString], for a brief description of the object.
   ///  * [toStringDeep], for a description of the subtree rooted at this object.
-  String? toStringShallow({
+  String toStringShallow({
     String joiner = ', ',
     DiagnosticLevel minLevel = DiagnosticLevel.debug,
   }) {
-    if (kReleaseMode) {
-      return toString();
-    }
     String? shallowString;
     assert(() {
       final StringBuffer result = StringBuffer();
@@ -3398,7 +3395,7 @@ abstract class DiagnosticableTree with Diagnosticable {
       shallowString = result.toString();
       return true;
     }());
-    return shallowString;
+    return shallowString ?? toString();
   }
 
   /// Returns a string representation of this node and its descendants.
@@ -3470,13 +3467,10 @@ mixin DiagnosticableTreeMixin implements DiagnosticableTree {
   }
 
   @override
-  String? toStringShallow({
+  String toStringShallow({
     String joiner = ', ',
     DiagnosticLevel minLevel = DiagnosticLevel.debug,
   }) {
-    if (kReleaseMode) {
-      return toString();
-    }
     String? shallowString;
     assert(() {
       final StringBuffer result = StringBuffer();
@@ -3491,7 +3485,7 @@ mixin DiagnosticableTreeMixin implements DiagnosticableTree {
       shallowString = result.toString();
       return true;
     }());
-    return shallowString;
+    return shallowString ?? toString();
   }
 
   @override

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

@@ -291,6 +291,7 @@ mixin GestureBinding on BindingBase implements HitTestable, HitTestDispatcher, H
         event is PointerHoverEvent ||
         event is PointerAddedEvent ||
         event is PointerRemovedEvent) {
+      assert(event.position != null);
       dispatchEvent(event, hitTestResult);
     }
   }

packages/flutter/lib/src/gestures/converter.dart

@@ -48,6 +48,7 @@ class PointerEventConverter {
   static Iterable<PointerEvent> expand(Iterable<ui.PointerData> data, double devicePixelRatio) sync* {
     for (final ui.PointerData datum in data) {
       final Offset position = Offset(datum.physicalX, datum.physicalY) / devicePixelRatio;
+      assert(position != null);
       final Offset delta = Offset(datum.physicalDeltaX, datum.physicalDeltaY) / devicePixelRatio;
       final double radiusMinor = _toLogicalPixels(datum.radiusMinor, devicePixelRatio);
       final double radiusMajor = _toLogicalPixels(datum.radiusMajor, devicePixelRatio);

packages/flutter/lib/src/painting/text_painter.dart

@@ -44,6 +44,9 @@ class PlaceholderDimensions {
   }) : assert(size != null),
        assert(alignment != null);
 
+  /// A constant representing an empty placeholder.
+  static const PlaceholderDimensions empty = PlaceholderDimensions(size: Size.zero, alignment: ui.PlaceholderAlignment.bottom);
+
   /// Width and height dimensions of the placeholder.
   final Size size;
 

packages/flutter/lib/src/rendering/animated_size.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/animation.dart';
 import 'package:flutter/foundation.dart';
 import 'package:flutter/scheduler.dart';
@@ -76,13 +74,13 @@ class RenderAnimatedSize extends RenderAligningShiftedBox {
   /// The arguments [duration], [curve], [alignment], and [vsync] must
   /// not be null.
   RenderAnimatedSize({
-    @required TickerProvider vsync,
-    @required Duration duration,
-    Duration reverseDuration,
+    required TickerProvider vsync,
+    required Duration duration,
+    Duration? reverseDuration,
     Curve curve = Curves.linear,
     AlignmentGeometry alignment = Alignment.center,
-    TextDirection textDirection,
-    RenderBox child,
+    TextDirection? textDirection,
+    RenderBox? child,
   }) : assert(vsync != null),
        assert(duration != null),
        assert(curve != null),
@@ -102,11 +100,11 @@ class RenderAnimatedSize extends RenderAligningShiftedBox {
     );
   }
 
-  AnimationController _controller;
-  CurvedAnimation _animation;
+  late final AnimationController _controller;
+  late final CurvedAnimation _animation;
   final SizeTween _sizeTween = SizeTween();
-  bool _hasVisualOverflow;
-  double _lastValue;
+  late bool _hasVisualOverflow;
+  double? _lastValue;
 
   /// The state this size animation is in.
   ///
@@ -116,7 +114,7 @@ class RenderAnimatedSize extends RenderAligningShiftedBox {
   RenderAnimatedSizeState _state = RenderAnimatedSizeState.start;
 
   /// The duration of the animation.
-  Duration get duration => _controller.duration;
+  Duration get duration => _controller.duration!;
   set duration(Duration value) {
     assert(value != null);
     if (value == _controller.duration)
@@ -125,8 +123,8 @@ class RenderAnimatedSize extends RenderAligningShiftedBox {
   }
 
   /// The duration of the animation when running in reverse.
-  Duration get reverseDuration => _controller.reverseDuration;
-  set reverseDuration(Duration value) {
+  Duration? get reverseDuration => _controller.reverseDuration;
+  set reverseDuration(Duration? value) {
     if (value == _controller.reverseDuration)
       return;
     _controller.reverseDuration = value;
@@ -164,7 +162,7 @@ class RenderAnimatedSize extends RenderAligningShiftedBox {
     super.detach();
   }
 
-  Size get _animatedSize {
+  Size? get _animatedSize {
     return _sizeTween.evaluate(_animation);
   }
 
@@ -181,7 +179,7 @@ class RenderAnimatedSize extends RenderAligningShiftedBox {
       return;
     }
 
-    child.layout(constraints, parentUsesSize: true);
+    child!.layout(constraints, parentUsesSize: true);
 
     assert(_state != null);
     switch (_state) {
@@ -199,11 +197,11 @@ class RenderAnimatedSize extends RenderAligningShiftedBox {
         break;
     }
 
-    size = constraints.constrain(_animatedSize);
+    size = constraints.constrain(_animatedSize!);
     alignChild();
 
-    if (size.width < _sizeTween.end.width ||
-        size.height < _sizeTween.end.height)
+    if (size.width < _sizeTween.end!.width ||
+        size.height < _sizeTween.end!.height)
       _hasVisualOverflow = true;
   }
 
@@ -217,7 +215,7 @@ class RenderAnimatedSize extends RenderAligningShiftedBox {
   /// We have the initial size to animate from, but we do not have the target
   /// size to animate to, so we set both ends to child's size.
   void _layoutStart() {
-    _sizeTween.begin = _sizeTween.end = debugAdoptSize(child.size);
+    _sizeTween.begin = _sizeTween.end = debugAdoptSize(child!.size);
     _state = RenderAnimatedSizeState.stable;
   }
 
@@ -227,14 +225,14 @@ class RenderAnimatedSize extends RenderAligningShiftedBox {
   /// If during animation the size of the child changes we restart the
   /// animation.
   void _layoutStable() {
-    if (_sizeTween.end != child.size) {
+    if (_sizeTween.end != child!.size) {
       _sizeTween.begin = size;
-      _sizeTween.end = debugAdoptSize(child.size);
+      _sizeTween.end = debugAdoptSize(child!.size);
       _restartAnimation();
       _state = RenderAnimatedSizeState.changed;
     } else if (_controller.value == _controller.upperBound) {
       // Animation finished. Reset target sizes.
-      _sizeTween.begin = _sizeTween.end = debugAdoptSize(child.size);
+      _sizeTween.begin = _sizeTween.end = debugAdoptSize(child!.size);
     } else if (!_controller.isAnimating) {
       _controller.forward(); // resume the animation after being detached
     }
@@ -247,9 +245,9 @@ class RenderAnimatedSize extends RenderAligningShiftedBox {
   /// changes again, we match the child's size, restart animation and go to
   /// unstable state.
   void _layoutChanged() {
-    if (_sizeTween.end != child.size) {
+    if (_sizeTween.end != child!.size) {
       // Child size changed again. Match the child's size and restart animation.
-      _sizeTween.begin = _sizeTween.end = debugAdoptSize(child.size);
+      _sizeTween.begin = _sizeTween.end = debugAdoptSize(child!.size);
       _restartAnimation();
       _state = RenderAnimatedSizeState.unstable;
     } else {
@@ -264,9 +262,9 @@ class RenderAnimatedSize extends RenderAligningShiftedBox {
   ///
   /// Continue tracking the child's size until is stabilizes.
   void _layoutUnstable() {
-    if (_sizeTween.end != child.size) {
+    if (_sizeTween.end != child!.size) {
       // Still unstable. Continue tracking the child.
-      _sizeTween.begin = _sizeTween.end = debugAdoptSize(child.size);
+      _sizeTween.begin = _sizeTween.end = debugAdoptSize(child!.size);
       _restartAnimation();
     } else {
       // Child size stabilized.

packages/flutter/lib/src/rendering/binding.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:developer';
 import 'dart:typed_data';
@@ -50,8 +48,8 @@ mixin RendererBinding on BindingBase, ServicesBinding, SchedulerBinding, Gesture
   }
 
   /// The current [RendererBinding], if one has been created.
-  static RendererBinding get instance => _instance;
-  static RendererBinding _instance;
+  static RendererBinding? get instance => _instance;
+  static RendererBinding? _instance;
 
   @override
   void initServiceExtensions() {
@@ -145,20 +143,25 @@ mixin RendererBinding on BindingBase, ServicesBinding, SchedulerBinding, Gesture
   ///
   /// Called automatically when the binding is created.
   void initRenderView() {
-    assert(renderView == null);
+    assert(!_debugIsRenderViewInitialized);
+    assert(() {
+      _debugIsRenderViewInitialized = true;
+      return true;
+    }());
     renderView = RenderView(configuration: createViewConfiguration(), window: window);
     renderView.prepareInitialFrame();
   }
+  bool _debugIsRenderViewInitialized = false;
 
   /// The object that manages state about currently connected mice, for hover
   /// notification.
-  MouseTracker get mouseTracker => _mouseTracker;
-  MouseTracker _mouseTracker;
+  MouseTracker get mouseTracker => _mouseTracker!;
+  MouseTracker? _mouseTracker;
 
   /// The render tree's owner, which maintains dirty state for layout,
   /// composite, paint, and accessibility semantics.
   PipelineOwner get pipelineOwner => _pipelineOwner;
-  PipelineOwner _pipelineOwner;
+  late PipelineOwner _pipelineOwner;
 
   /// The render tree that's attached to the output surface.
   RenderView get renderView => _pipelineOwner.rootNode as RenderView;
@@ -239,25 +242,26 @@ mixin RendererBinding on BindingBase, ServicesBinding, SchedulerBinding, Gesture
     );
   }
 
-  SemanticsHandle _semanticsHandle;
+  SemanticsHandle? _semanticsHandle;
 
   /// Creates a [MouseTracker] which manages state about currently connected
   /// mice, for hover notification.
   ///
   /// Used by testing framework to reinitialize the mouse tracker between tests.
   @visibleForTesting
-  void initMouseTracker([MouseTracker tracker]) {
+  void initMouseTracker([MouseTracker? tracker]) {
     _mouseTracker?.dispose();
     _mouseTracker = tracker ?? MouseTracker();
   }
 
   @override // from GestureBinding
-  void dispatchEvent(PointerEvent event, HitTestResult hitTestResult) {
+  void dispatchEvent(PointerEvent event, HitTestResult? hitTestResult) {
     if (hitTestResult != null ||
         event is PointerHoverEvent ||
         event is PointerAddedEvent ||
         event is PointerRemovedEvent) {
-      _mouseTracker.updateWithEvent(event,
+      assert(event.position != null);
+      _mouseTracker!.updateWithEvent(event,
           () => hitTestResult ?? renderView.hitTestMouseTrackers(event.position));
     }
     super.dispatchEvent(event, hitTestResult);
@@ -278,7 +282,7 @@ mixin RendererBinding on BindingBase, ServicesBinding, SchedulerBinding, Gesture
     }
   }
 
-  void _handleSemanticsAction(int id, SemanticsAction action, ByteData args) {
+  void _handleSemanticsAction(int id, SemanticsAction action, ByteData? args) {
     _pipelineOwner.semanticsOwner?.performAction(
       id,
       action,
@@ -306,13 +310,13 @@ mixin RendererBinding on BindingBase, ServicesBinding, SchedulerBinding, Gesture
       _debugMouseTrackerUpdateScheduled = true;
       return true;
     }());
-    SchedulerBinding.instance.addPostFrameCallback((Duration duration) {
+    SchedulerBinding.instance!.addPostFrameCallback((Duration duration) {
       assert(_debugMouseTrackerUpdateScheduled);
       assert(() {
         _debugMouseTrackerUpdateScheduled = false;
         return true;
       }());
-      _mouseTracker.updateAllDevices(renderView.hitTestMouseTrackers);
+      _mouseTracker!.updateAllDevices(renderView.hitTestMouseTrackers);
     });
   }
 
@@ -455,29 +459,31 @@ mixin RendererBinding on BindingBase, ServicesBinding, SchedulerBinding, Gesture
   @override
   void hitTest(HitTestResult result, Offset position) {
     assert(renderView != null);
+    assert(result != null);
+    assert(position != null);
     renderView.hitTest(result, position: position);
     super.hitTest(result, position);
   }
 
   Future<void> _forceRepaint() {
-    RenderObjectVisitor visitor;
+    late RenderObjectVisitor visitor;
     visitor = (RenderObject child) {
       child.markNeedsPaint();
       child.visitChildren(visitor);
     };
-    instance?.renderView?.visitChildren(visitor);
+    instance?.renderView.visitChildren(visitor);
     return endOfFrame;
   }
 }
 
 /// Prints a textual representation of the entire render tree.
 void debugDumpRenderTree() {
-  debugPrint(RendererBinding.instance?.renderView?.toStringDeep() ?? 'Render tree unavailable.');
+  debugPrint(RendererBinding.instance?.renderView.toStringDeep() ?? 'Render tree unavailable.');
 }
 
 /// Prints a textual representation of the entire layer tree.
 void debugDumpLayerTree() {
-  debugPrint(RendererBinding.instance?.renderView?.debugLayer?.toStringDeep() ?? 'Layer tree unavailable.');
+  debugPrint(RendererBinding.instance?.renderView.debugLayer?.toStringDeep() ?? 'Layer tree unavailable.');
 }
 
 /// Prints a textual representation of the entire semantics tree.
@@ -487,7 +493,7 @@ void debugDumpLayerTree() {
 /// The order in which the children of a [SemanticsNode] will be printed is
 /// controlled by the [childOrder] parameter.
 void debugDumpSemanticsTree(DebugSemanticsDumpOrder childOrder) {
-  debugPrint(RendererBinding.instance?.renderView?.debugSemantics?.toStringDeep(childOrder: childOrder) ?? 'Semantics not collected.');
+  debugPrint(RendererBinding.instance?.renderView.debugSemantics?.toStringDeep(childOrder: childOrder) ?? 'Semantics not collected.');
 }
 
 /// A concrete binding for applications that use the Rendering framework
@@ -502,7 +508,7 @@ class RenderingFlutterBinding extends BindingBase with GestureBinding, Scheduler
   ///
   /// The `root` render box is attached directly to the [renderView] and is
   /// given constraints that require it to fill the window.
-  RenderingFlutterBinding({ RenderBox root }) {
+  RenderingFlutterBinding({ RenderBox? root }) {
     assert(renderView != null);
     renderView.child = root;
   }

packages/flutter/lib/src/rendering/custom_layout.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';
 
 import 'box.dart';
@@ -14,7 +12,7 @@ import 'object.dart';
 /// [ParentData] used by [RenderCustomMultiChildLayoutBox].
 class MultiChildLayoutParentData extends ContainerBoxParentData<RenderBox> {
   /// An object representing the identity of this child.
-  Object id;
+  Object? id;
 
   @override
   String toString() => '${super.toString()}; id=$id';
@@ -120,20 +118,19 @@ abstract class MultiChildLayoutDelegate {
   /// Creates a layout delegate.
   ///
   /// The layout will update whenever [relayout] notifies its listeners.
-  MultiChildLayoutDelegate({ Listenable relayout }) : _relayout = relayout;
+  MultiChildLayoutDelegate({ Listenable? relayout }) : _relayout = relayout;
 
-  final Listenable _relayout;
+  final Listenable? _relayout;
 
-  // TODO(ianh): make these late final
-  /*late*/ Map<Object/*!*/, RenderBox>/*!*/ _idToChild;
-  /*late*/ Set<RenderBox/*!*/>/*!*/ _debugChildrenNeedingLayout;
+  Map<Object, RenderBox>? _idToChild;
+  Set<RenderBox>? _debugChildrenNeedingLayout;
 
   /// True if a non-null LayoutChild was provided for the specified id.
   ///
   /// Call this from the [performLayout] or [getSize] methods to
   /// determine which children are available, if the child list might
   /// vary.
-  bool hasChild(Object childId) => _idToChild[childId] != null;
+  bool hasChild(Object childId) => _idToChild![childId] != null;
 
   /// Ask the child to update its layout within the limits specified by
   /// the constraints parameter. The child's size is returned.
@@ -141,8 +138,8 @@ abstract class MultiChildLayoutDelegate {
   /// Call this from your [performLayout] function to lay out each
   /// child. Every child must be laid out using this function exactly
   /// once each time the [performLayout] function is called.
-  Size/*!*/ layoutChild(Object childId, BoxConstraints constraints) {
-    final RenderBox child = _idToChild[childId];
+  Size layoutChild(Object childId, BoxConstraints constraints) {
+    final RenderBox? child = _idToChild![childId];
     assert(() {
       if (child == null) {
         throw FlutterError(
@@ -150,7 +147,7 @@ abstract class MultiChildLayoutDelegate {
           'There is no child with the id "$childId".'
         );
       }
-      if (!_debugChildrenNeedingLayout.remove(child)) {
+      if (!_debugChildrenNeedingLayout!.remove(child)) {
         throw FlutterError(
           'The $this custom multichild layout delegate tried to lay out the child with id "$childId" more than once.\n'
           'Each child must be laid out exactly once.'
@@ -171,7 +168,7 @@ abstract class MultiChildLayoutDelegate {
       }
       return true;
     }());
-    child.layout(constraints, parentUsesSize: true);
+    child!.layout(constraints, parentUsesSize: true);
     return child.size;
   }
 
@@ -182,7 +179,7 @@ abstract class MultiChildLayoutDelegate {
   /// remain unchanged. Children initially have their position set to
   /// (0,0), i.e. the top left of the [RenderCustomMultiChildLayoutBox].
   void positionChild(Object childId, Offset offset) {
-    final RenderBox child = _idToChild[childId];
+    final RenderBox? child = _idToChild![childId];
     assert(() {
       if (child == null) {
         throw FlutterError(
@@ -190,14 +187,17 @@ abstract class MultiChildLayoutDelegate {
           'There is no child with the id "$childId".'
         );
       }
-      if (offset == null) {
+      // `offset` has a non-nullable return type, but might be null when
+      // running with weak checking, so we need to null check it anyway (and
+      // ignore the warning that the null-handling logic is dead code).
+      if (offset == null) { // ignore: dead_code
         throw FlutterError(
           'The $this custom multichild layout delegate provided a null position for the child with id "$childId".'
         );
       }
       return true;
     }());
-    final MultiChildLayoutParentData childParentData = child.parentData as MultiChildLayoutParentData;
+    final MultiChildLayoutParentData childParentData = child!.parentData as MultiChildLayoutParentData;
     childParentData.offset = offset;
   }
 
@@ -206,14 +206,13 @@ abstract class MultiChildLayoutDelegate {
     return DiagnosticsProperty<RenderBox>('${childParentData.id}', child);
   }
 
-  void _callPerformLayout(Size size, RenderBox firstChild) {
+  void _callPerformLayout(Size size, RenderBox? firstChild) {
     // A particular layout delegate could be called reentrantly, e.g. if it used
     // by both a parent and a child. So, we must restore the _idToChild map when
     // we return.
-    final Map<Object, RenderBox> previousIdToChild = _idToChild;
+    final Map<Object, RenderBox>? previousIdToChild = _idToChild;
 
-    // TODO(ianh): make the next line final
-    /*late*/ Set<RenderBox>/*!*/ debugPreviousChildrenNeedingLayout;
+    Set<RenderBox>? debugPreviousChildrenNeedingLayout;
     assert(() {
       debugPreviousChildrenNeedingLayout = _debugChildrenNeedingLayout;
       _debugChildrenNeedingLayout = <RenderBox>{};
@@ -222,36 +221,36 @@ abstract class MultiChildLayoutDelegate {
 
     try {
       _idToChild = <Object, RenderBox>{};
-      RenderBox child = firstChild;
+      RenderBox? child = firstChild;
       while (child != null) {
         final MultiChildLayoutParentData childParentData = child.parentData as MultiChildLayoutParentData;
         assert(() {
           if (childParentData.id == null) {
             throw FlutterError.fromParts(<DiagnosticsNode>[
               ErrorSummary('Every child of a RenderCustomMultiChildLayoutBox must have an ID in its parent data.'),
-              child.describeForError('The following child has no ID'),
+              child!.describeForError('The following child has no ID'),
             ]);
           }
           return true;
         }());
-        _idToChild[childParentData.id] = child;
+        _idToChild![childParentData.id!] = child;
         assert(() {
-          _debugChildrenNeedingLayout.add(child);
+          _debugChildrenNeedingLayout!.add(child!);
           return true;
         }());
         child = childParentData.nextSibling;
       }
       performLayout(size);
       assert(() {
-        if (_debugChildrenNeedingLayout.isNotEmpty) {
+        if (_debugChildrenNeedingLayout!.isNotEmpty) {
           throw FlutterError.fromParts(<DiagnosticsNode>[
           ErrorSummary('Each child must be laid out exactly once.'),
             DiagnosticsBlock(
               name:
                 'The $this custom multichild layout delegate forgot '
                 'to lay out the following '
-                '${_debugChildrenNeedingLayout.length > 1 ? 'children' : 'child'}',
-              properties: _debugChildrenNeedingLayout.map<DiagnosticsNode>(_debugDescribeChild).toList(),
+                '${_debugChildrenNeedingLayout!.length > 1 ? 'children' : 'child'}',
+              properties: _debugChildrenNeedingLayout!.map<DiagnosticsNode>(_debugDescribeChild).toList(),
               style: DiagnosticsTreeStyle.whitespace,
             ),
           ]);
@@ -314,8 +313,8 @@ class RenderCustomMultiChildLayoutBox extends RenderBox
   ///
   /// The [delegate] argument must not be null.
   RenderCustomMultiChildLayoutBox({
-    List<RenderBox> children,
-    @required MultiChildLayoutDelegate delegate,
+    List<RenderBox>? children,
+    required MultiChildLayoutDelegate delegate,
   }) : assert(delegate != null),
        _delegate = delegate {
     addAll(children);
@@ -339,20 +338,20 @@ class RenderCustomMultiChildLayoutBox extends RenderBox
       markNeedsLayout();
     _delegate = newDelegate;
     if (attached) {
-      oldDelegate?._relayout?.removeListener(markNeedsLayout);
-      newDelegate?._relayout?.addListener(markNeedsLayout);
+      oldDelegate._relayout?.removeListener(markNeedsLayout);
+      newDelegate._relayout?.addListener(markNeedsLayout);
     }
   }
 
   @override
   void attach(PipelineOwner owner) {
     super.attach(owner);
-    _delegate?._relayout?.addListener(markNeedsLayout);
+    _delegate._relayout?.addListener(markNeedsLayout);
   }
 
   @override
   void detach() {
-    _delegate?._relayout?.removeListener(markNeedsLayout);
+    _delegate._relayout?.removeListener(markNeedsLayout);
     super.detach();
   }
 
@@ -409,7 +408,7 @@ class RenderCustomMultiChildLayoutBox extends RenderBox
   }
 
   @override
-  bool hitTestChildren(BoxHitTestResult result, { Offset position }) {
+  bool hitTestChildren(BoxHitTestResult result, { required Offset position }) {
     return defaultHitTestChildren(result, position: position);
   }
 }

packages/flutter/lib/src/rendering/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';
 import 'package:flutter/painting.dart';
 
@@ -88,6 +86,11 @@ bool debugRepaintTextRainbowEnabled = false;
 ///
 /// This check helps developers that want a consistent look and feel detect
 /// where this inconsistency would occur.
+///
+/// This check assumes that elevations on [PhysicalModelLayer] objects have
+/// been set to non-null values before the scene is built. If this assumption
+/// is violated, the check will throw exceptions. (The scene building would
+/// also fail in that case, however.)
 bool debugCheckElevationsEnabled = false;
 
 /// The current color to overlay when repainting a layer.
@@ -174,7 +177,7 @@ bool debugProfileLayoutsEnabled = false;
 bool debugProfilePaintsEnabled = false;
 
 /// Signature for [debugOnProfilePaint] implementations.
-typedef ProfilePaintCallback = void Function(RenderObject/*!*/ renderObject);
+typedef ProfilePaintCallback = void Function(RenderObject renderObject);
 
 /// Callback invoked for every [RenderObject] painted each frame.
 ///
@@ -190,7 +193,7 @@ typedef ProfilePaintCallback = void Function(RenderObject/*!*/ renderObject);
 ///    callback to generate aggregate profile statistics describing what paints
 ///    occurred when the `ext.flutter.inspector.trackRepaintWidgets` service
 ///    extension is enabled.
-ProfilePaintCallback debugOnProfilePaint;
+ProfilePaintCallback? debugOnProfilePaint;
 
 /// Setting to true will cause all clipping effects from the layer tree to be
 /// ignored.
@@ -243,7 +246,7 @@ void _debugDrawDoubleRect(Canvas canvas, Rect outerRect, Rect innerRect, Color c
 ///
 /// Called by [RenderPadding.debugPaintSize] when [debugPaintSizeEnabled] is
 /// true.
-void debugPaintPadding(Canvas canvas, Rect outerRect, Rect innerRect, { double outlineWidth = 2.0 }) {
+void debugPaintPadding(Canvas canvas, Rect outerRect, Rect? innerRect, { double outlineWidth = 2.0 }) {
   assert(() {
     if (innerRect != null && !innerRect.isEmpty) {
       _debugDrawDoubleRect(canvas, outerRect, innerRect, const Color(0x900090FF));

packages/flutter/lib/src/rendering/debug_overflow_indicator.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:math' as math;
 import 'dart:ui' as ui;
 
@@ -25,11 +23,11 @@ enum _OverflowSide {
 // the indicators.
 class _OverflowRegionData {
   const _OverflowRegionData({
-    this.rect,
+    required this.rect,
     this.label = '',
     this.labelOffset = Offset.zero,
     this.rotation = 0.0,
-    this.side,
+    required this.side,
   });
 
   final Rect rect;
@@ -201,7 +199,7 @@ mixin DebugOverflowIndicatorMixin on RenderObject {
     return regions;
   }
 
-  void _reportOverflow(RelativeRect overflow, List<DiagnosticsNode> overflowHints) {
+  void _reportOverflow(RelativeRect overflow, List<DiagnosticsNode>? overflowHints) {
     overflowHints ??= <DiagnosticsNode>[];
     if (overflowHints.isEmpty) {
       overflowHints.add(ErrorDescription(
@@ -248,8 +246,8 @@ mixin DebugOverflowIndicatorMixin on RenderObject {
         context: ErrorDescription('during layout'),
         informationCollector: () sync* {
           if (debugCreator != null)
-            yield DiagnosticsDebugCreator(debugCreator);
-          yield* overflowHints;
+            yield DiagnosticsDebugCreator(debugCreator as Object);
+          yield* overflowHints!;
           yield describeForError('The specific $runtimeType in question is');
           // TODO(jacobr): this line is ascii art that it would be nice to
           // handle a little more generically in GUI debugging clients in the
@@ -271,7 +269,7 @@ mixin DebugOverflowIndicatorMixin on RenderObject {
     Offset offset,
     Rect containerRect,
     Rect childRect, {
-    List<DiagnosticsNode> overflowHints,
+    List<DiagnosticsNode>? overflowHints,
   }) {
     final RelativeRect overflow = RelativeRect.fromRect(containerRect, childRect);
 
@@ -285,7 +283,7 @@ mixin DebugOverflowIndicatorMixin on RenderObject {
     final List<_OverflowRegionData> overflowRegions = _calculateOverflowRegions(overflow, containerRect);
     for (final _OverflowRegionData region in overflowRegions) {
       context.canvas.drawRect(region.rect.shift(offset), _indicatorPaint);
-      final TextSpan textSpan = _indicatorLabel[region.side.index].text as TextSpan;
+      final TextSpan? textSpan = _indicatorLabel[region.side.index].text as TextSpan?;
       if (textSpan?.text != region.label) {
         _indicatorLabel[region.side.index].text = TextSpan(
           text: region.label,

packages/flutter/lib/src/rendering/error.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:ui' as ui show Paragraph, ParagraphBuilder, ParagraphConstraints, ParagraphStyle, TextStyle;
 
 import 'box.dart';
@@ -59,7 +57,7 @@ class RenderErrorBox extends RenderBox {
   final String message;
 
   // TODO(ianh): should be final
-  /*late*/ ui.Paragraph/*?*/ _paragraph;
+  ui.Paragraph? _paragraph;
 
   @override
   double computeMaxIntrinsicWidth(double height) {
@@ -158,11 +156,11 @@ class RenderErrorBox extends RenderBox {
           width -= padding.left + padding.right;
           left += padding.left;
         }
-        _paragraph.layout(ui.ParagraphConstraints(width: width));
-        if (size.height > padding.top + _paragraph.height + padding.bottom) {
+        _paragraph!.layout(ui.ParagraphConstraints(width: width));
+        if (size.height > padding.top + _paragraph!.height + padding.bottom) {
           top += padding.top;
         }
-        context.canvas.drawParagraph(_paragraph, offset + Offset(left, top));
+        context.canvas.drawParagraph(_paragraph!, offset + Offset(left, top));
       }
     } catch (e) {
       // Intentionally left empty.

packages/flutter/lib/src/rendering/flow.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:ui' as ui show Color;
 
 import 'package:flutter/foundation.dart';
@@ -32,7 +30,7 @@ abstract class FlowPaintingContext {
   /// The size of the [i]th child.
   ///
   /// If [i] is negative or exceeds [childCount], returns null.
-  Size getChildSize(int i);
+  Size? getChildSize(int i);
 
   /// Paint the [i]th child using the given transform.
   ///
@@ -59,9 +57,9 @@ abstract class FlowPaintingContext {
 ///  * [RenderFlow]
 abstract class FlowDelegate {
   /// The flow will repaint whenever [repaint] notifies its listeners.
-  const FlowDelegate({ Listenable repaint }) : _repaint = repaint;
+  const FlowDelegate({ Listenable? repaint }) : _repaint = repaint;
 
-  final Listenable _repaint;
+  final Listenable? _repaint;
 
   /// Override to control the size of the container for the children.
   ///
@@ -145,7 +143,7 @@ abstract class FlowDelegate {
 /// matrix, use the [FlowPaintingContext.paintChild] function from an override
 /// of the [FlowDelegate.paintChildren] function.
 class FlowParentData extends ContainerBoxParentData<RenderBox> {
-  Matrix4 _transform;
+  Matrix4? _transform;
 }
 
 /// Implements the flow layout algorithm.
@@ -181,8 +179,8 @@ class RenderFlow extends RenderBox
   /// For optimal performance, consider using children that return true from
   /// [isRepaintBoundary].
   RenderFlow({
-    List<RenderBox> children,
-    @required FlowDelegate delegate,
+    List<RenderBox>? children,
+    required FlowDelegate delegate,
   }) : assert(delegate != null),
        _delegate = delegate {
     addAll(children);
@@ -190,7 +188,7 @@ class RenderFlow extends RenderBox
 
   @override
   void setupParentData(RenderBox child) {
-    final ParentData childParentData = child.parentData;
+    final ParentData? childParentData = child.parentData;
     if (childParentData is FlowParentData)
       childParentData._transform = null;
     else
@@ -285,7 +283,7 @@ class RenderFlow extends RenderBox
     size = _getSize(constraints);
     int i = 0;
     _randomAccessChildren.clear();
-    RenderBox child = firstChild;
+    RenderBox? child = firstChild;
     while (child != null) {
       _randomAccessChildren.add(child);
       final BoxConstraints innerConstraints = _delegate.getConstraintsForChild(i, constraints);
@@ -304,18 +302,18 @@ class RenderFlow extends RenderBox
   final List<int> _lastPaintOrder = <int>[];
 
   // Only valid during paint.
-  PaintingContext _paintingContext;
-  Offset _paintingOffset;
+  PaintingContext? _paintingContext;
+  Offset? _paintingOffset;
 
   @override
-  Size getChildSize(int i) {
+  Size? getChildSize(int i) {
     if (i < 0 || i >= _randomAccessChildren.length)
       return null;
     return _randomAccessChildren[i].size;
   }
 
   @override
-  void paintChild(int i, { Matrix4 transform, double opacity = 1.0 }) {
+  void paintChild(int i, { Matrix4? transform, double opacity = 1.0 }) {
     transform ??= Matrix4.identity();
     final RenderBox child = _randomAccessChildren[i];
     final FlowParentData childParentData = child.parentData as FlowParentData;
@@ -341,10 +339,10 @@ class RenderFlow extends RenderBox
       context.paintChild(child, offset);
     }
     if (opacity == 1.0) {
-      _paintingContext.pushTransform(needsCompositing, _paintingOffset, transform, painter);
+      _paintingContext!.pushTransform(needsCompositing, _paintingOffset!, transform, painter);
     } else {
-      _paintingContext.pushOpacity(_paintingOffset, ui.Color.getAlphaFromOpacity(opacity), (PaintingContext context, Offset offset) {
-        context.pushTransform(needsCompositing, offset, transform, painter);
+      _paintingContext!.pushOpacity(_paintingOffset!, ui.Color.getAlphaFromOpacity(opacity), (PaintingContext context, Offset offset) {
+        context.pushTransform(needsCompositing, offset, transform!, painter);
       });
     }
   }
@@ -371,7 +369,7 @@ class RenderFlow extends RenderBox
   }
 
   @override
-  bool hitTestChildren(BoxHitTestResult result, { Offset position }) {
+  bool hitTestChildren(BoxHitTestResult result, { required Offset position }) {
     final List<RenderBox> children = getChildrenAsList();
     for (int i = _lastPaintOrder.length - 1; i >= 0; --i) {
       final int childIndex = _lastPaintOrder[i];
@@ -379,14 +377,14 @@ class RenderFlow extends RenderBox
         continue;
       final RenderBox child = children[childIndex];
       final FlowParentData childParentData = child.parentData as FlowParentData;
-      final Matrix4 transform = childParentData._transform;
+      final Matrix4? transform = childParentData._transform;
       if (transform == null)
         continue;
       final bool absorbed = result.addWithPaintTransform(
         transform: transform,
         position: position,
-        hitTest: (BoxHitTestResult result, Offset position) {
-          return child.hitTest(result, position: position);
+        hitTest: (BoxHitTestResult result, Offset? position) {
+          return child.hitTest(result, position: position!);
         },
       );
       if (absorbed)
@@ -399,7 +397,7 @@ class RenderFlow extends RenderBox
   void applyPaintTransform(RenderBox child, Matrix4 transform) {
     final FlowParentData childParentData = child.parentData as FlowParentData;
     if (childParentData._transform != null)
-      transform.multiply(childParentData._transform);
+      transform.multiply(childParentData._transform!);
     super.applyPaintTransform(child, transform);
   }
 }

packages/flutter/lib/src/rendering/image.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:ui' as ui show Image;
 
 import 'box.dart';
@@ -27,19 +25,19 @@ class RenderImage extends RenderBox {
   /// must not be null. The [textDirection] argument must not be null if
   /// [alignment] will need resolving or if [matchTextDirection] is true.
   RenderImage({
-    ui.Image image,
+    ui.Image? image,
     this.debugImageLabel,
-    double width,
-    double height,
+    double? width,
+    double? height,
     double scale = 1.0,
-    Color color,
-    BlendMode colorBlendMode,
-    BoxFit fit,
+    Color? color,
+    BlendMode? colorBlendMode,
+    BoxFit? fit,
     AlignmentGeometry alignment = Alignment.center,
     ImageRepeat repeat = ImageRepeat.noRepeat,
-    Rect centerSlice,
+    Rect? centerSlice,
     bool matchTextDirection = false,
-    TextDirection textDirection,
+    TextDirection? textDirection,
     bool invertColors = false,
     bool isAntiAlias = false,
     FilterQuality filterQuality = FilterQuality.low,
@@ -67,8 +65,8 @@ class RenderImage extends RenderBox {
     _updateColorFilter();
   }
 
-  Alignment _resolvedAlignment;
-  bool _flipHorizontally;
+  Alignment? _resolvedAlignment;
+  bool? _flipHorizontally;
 
   void _resolve() {
     if (_resolvedAlignment != null)
@@ -84,9 +82,9 @@ class RenderImage extends RenderBox {
   }
 
   /// The image to display.
-  ui.Image get image => _image;
-  ui.Image _image;
-  set image(ui.Image value) {
+  ui.Image? get image => _image;
+  ui.Image? _image;
+  set image(ui.Image? value) {
     if (value == _image)
       return;
     _image = value;
@@ -96,15 +94,15 @@ class RenderImage extends RenderBox {
   }
 
   /// A string used to identify the source of the image.
-  String debugImageLabel;
+  String? debugImageLabel;
 
   /// If non-null, requires the image to have this width.
   ///
   /// If null, the image will pick a size that best preserves its intrinsic
   /// aspect ratio.
-  double get width => _width;
-  double _width;
-  set width(double value) {
+  double? get width => _width;
+  double? _width;
+  set width(double? value) {
     if (value == _width)
       return;
     _width = value;
@@ -115,9 +113,9 @@ class RenderImage extends RenderBox {
   ///
   /// If null, the image will pick a size that best preserves its intrinsic
   /// aspect ratio.
-  double get height => _height;
-  double _height;
-  set height(double value) {
+  double? get height => _height;
+  double? _height;
+  set height(double? value) {
     if (value == _height)
       return;
     _height = value;
@@ -137,19 +135,19 @@ class RenderImage extends RenderBox {
     markNeedsLayout();
   }
 
-  ColorFilter _colorFilter;
+  ColorFilter? _colorFilter;
 
   void _updateColorFilter() {
     if (_color == null)
       _colorFilter = null;
     else
-      _colorFilter = ColorFilter.mode(_color, _colorBlendMode ?? BlendMode.srcIn);
+      _colorFilter = ColorFilter.mode(_color!, _colorBlendMode ?? BlendMode.srcIn);
   }
 
   /// If non-null, this color is blended with each image pixel using [colorBlendMode].
-  Color get color => _color;
-  Color _color;
-  set color(Color value) {
+  Color? get color => _color;
+  Color? _color;
+  set color(Color? value) {
     if (value == _color)
       return;
     _color = value;
@@ -180,9 +178,9 @@ class RenderImage extends RenderBox {
   /// See also:
   ///
   ///  * [BlendMode], which includes an illustration of the effect of each blend mode.
-  BlendMode get colorBlendMode => _colorBlendMode;
-  BlendMode _colorBlendMode;
-  set colorBlendMode(BlendMode value) {
+  BlendMode? get colorBlendMode => _colorBlendMode;
+  BlendMode? _colorBlendMode;
+  set colorBlendMode(BlendMode? value) {
     if (value == _colorBlendMode)
       return;
     _colorBlendMode = value;
@@ -194,9 +192,9 @@ class RenderImage extends RenderBox {
   ///
   /// The default varies based on the other fields. See the discussion at
   /// [paintImage].
-  BoxFit get fit => _fit;
-  BoxFit _fit;
-  set fit(BoxFit value) {
+  BoxFit? get fit => _fit;
+  BoxFit? _fit;
+  set fit(BoxFit? value) {
     if (value == _fit)
       return;
     _fit = value;
@@ -235,9 +233,9 @@ class RenderImage extends RenderBox {
   /// region of the image above and below the center slice will be stretched
   /// only horizontally and the region of the image to the left and right of
   /// the center slice will be stretched only vertically.
-  Rect get centerSlice => _centerSlice;
-  Rect _centerSlice;
-  set centerSlice(Rect value) {
+  Rect? get centerSlice => _centerSlice;
+  Rect? _centerSlice;
+  set centerSlice(Rect? value) {
     if (value == _centerSlice)
       return;
     _centerSlice = value;
@@ -287,9 +285,9 @@ class RenderImage extends RenderBox {
   /// This may be changed to null, but only after the [alignment] and
   /// [matchTextDirection] properties have been changed to values that do not
   /// depend on the direction.
-  TextDirection get textDirection => _textDirection;
-  TextDirection _textDirection;
-  set textDirection(TextDirection value) {
+  TextDirection? get textDirection => _textDirection;
+  TextDirection? _textDirection;
+  set textDirection(TextDirection? value) {
     if (_textDirection == value)
       return;
     _textDirection = value;
@@ -329,8 +327,8 @@ class RenderImage extends RenderBox {
       return constraints.smallest;
 
     return constraints.constrainSizeAndAttemptToPreserveAspectRatio(Size(
-      _image.width.toDouble() / _scale,
-      _image.height.toDouble() / _scale,
+      _image!.width.toDouble() / _scale,
+      _image!.height.toDouble() / _scale,
     ));
   }
 
@@ -380,15 +378,15 @@ class RenderImage extends RenderBox {
     paintImage(
       canvas: context.canvas,
       rect: offset & size,
-      image: _image,
+      image: _image!,
       debugImageLabel: debugImageLabel,
       scale: _scale,
       colorFilter: _colorFilter,
       fit: _fit,
-      alignment: _resolvedAlignment,
+      alignment: _resolvedAlignment!,
       centerSlice: _centerSlice,
       repeat: _repeat,
-      flipHorizontally: _flipHorizontally,
+      flipHorizontally: _flipHorizontally!,
       invertColors: invertColors,
       filterQuality: _filterQuality,
       isAntiAlias: _isAntiAlias,

packages/flutter/lib/src/rendering/list_body.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:math' as math;
 
 import 'box.dart';
@@ -32,7 +30,7 @@ class RenderListBody extends RenderBox
   ///
   /// By default, children are arranged along the vertical axis.
   RenderListBody({
-    List<RenderBox> children,
+    List<RenderBox>? children,
     AxisDirection axisDirection = AxisDirection.down,
   }) : assert(axisDirection != null),
        _axisDirection = axisDirection {
@@ -123,7 +121,7 @@ class RenderListBody extends RenderBox
       ]);
     }());
     double mainAxisExtent = 0.0;
-    RenderBox child = firstChild;
+    RenderBox? child = firstChild;
     switch (axisDirection) {
       case AxisDirection.right:
         final BoxConstraints innerConstraints = BoxConstraints.tightFor(height: constraints.maxHeight);
@@ -201,7 +199,7 @@ class RenderListBody extends RenderBox
 
   double _getIntrinsicCrossAxis(_ChildSizingFunction childSize) {
     double extent = 0.0;
-    RenderBox child = firstChild;
+    RenderBox? child = firstChild;
     while (child != null) {
       extent = math.max(extent, childSize(child));
       final ListBodyParentData childParentData = child.parentData as ListBodyParentData;
@@ -212,7 +210,7 @@ class RenderListBody extends RenderBox
 
   double _getIntrinsicMainAxis(_ChildSizingFunction childSize) {
     double extent = 0.0;
-    RenderBox child = firstChild;
+    RenderBox? child = firstChild;
     while (child != null) {
       extent += childSize(child);
       final ListBodyParentData childParentData = child.parentData as ListBodyParentData;
@@ -230,7 +228,6 @@ class RenderListBody extends RenderBox
       case Axis.vertical:
         return _getIntrinsicCrossAxis((RenderBox child) => child.getMinIntrinsicWidth(height));
     }
-    return null;
   }
 
   @override
@@ -242,7 +239,6 @@ class RenderListBody extends RenderBox
       case Axis.vertical:
         return _getIntrinsicCrossAxis((RenderBox child) => child.getMaxIntrinsicWidth(height));
     }
-    return null;
   }
 
   @override
@@ -254,7 +250,6 @@ class RenderListBody extends RenderBox
       case Axis.vertical:
         return _getIntrinsicCrossAxis((RenderBox child) => child.getMinIntrinsicHeight(width));
     }
-    return null;
   }
 
   @override
@@ -266,11 +261,10 @@ class RenderListBody extends RenderBox
       case Axis.vertical:
         return _getIntrinsicCrossAxis((RenderBox child) => child.getMaxIntrinsicHeight(width));
     }
-    return null;
   }
 
   @override
-  double computeDistanceToActualBaseline(TextBaseline baseline) {
+  double? computeDistanceToActualBaseline(TextBaseline baseline) {
     return defaultComputeDistanceToFirstActualBaseline(baseline);
   }
 
@@ -280,7 +274,7 @@ class RenderListBody extends RenderBox
   }
 
   @override
-  bool hitTestChildren(BoxHitTestResult result, { Offset position }) {
+  bool hitTestChildren(BoxHitTestResult result, { required Offset position }) {
     return defaultHitTestChildren(result, position: position);
   }
 

packages/flutter/lib/src/rendering/list_wheel_viewport.dart

@@ -2,12 +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
-
 import 'dart:math' as math;
 
 import 'package:flutter/animation.dart';
-import 'package:flutter/foundation.dart';
 import 'package:vector_math/vector_math_64.dart' show Matrix4;
 
 import 'box.dart';
@@ -30,7 +27,7 @@ abstract class ListWheelChildManager {
   /// If null, then there's no explicit limits to the range of the children
   /// except that it has to be contiguous. If [childExistsAt] for a certain
   /// index returns false, that index is already past the limit.
-  int get childCount;
+  int? get childCount;
 
   /// Checks whether the delegate is able to provide a child widget at the given
   /// index.
@@ -44,7 +41,7 @@ abstract class ListWheelChildManager {
   /// nothing.
   ///
   /// It is possible to create children with negative indices.
-  void createChild(int index, { @required RenderBox after });
+  void createChild(int index, { required RenderBox? after });
 
   /// Removes the child element corresponding with the given RenderBox.
   void removeChild(RenderBox child);
@@ -53,7 +50,9 @@ abstract class ListWheelChildManager {
 /// [ParentData] for use with [RenderListWheelViewport].
 class ListWheelParentData extends ContainerBoxParentData<RenderBox> {
   /// Index of this child in its parent's child list.
-  int index;
+  ///
+  /// This must be maintained by the [ListWheelChildManager].
+  int? index;
 }
 
 /// Render, onto a wheel, a bigger sequential set of objects inside this viewport.
@@ -135,19 +134,19 @@ class RenderListWheelViewport
   ///
   /// All arguments must not be null. Optional arguments have reasonable defaults.
   RenderListWheelViewport({
-    @required this.childManager,
-    @required ViewportOffset offset,
+    required this.childManager,
+    required ViewportOffset offset,
     double diameterRatio = defaultDiameterRatio,
     double perspective = defaultPerspective,
     double offAxisFraction = 0,
     bool useMagnifier = false,
     double magnification = 1,
     double overAndUnderCenterOpacity = 1,
-    @required double itemExtent,
+    required double itemExtent,
     double squeeze = 1,
     bool renderChildrenOutsideViewport = false,
     Clip clipBehavior = Clip.none,
-    List<RenderBox> children,
+    List<RenderBox>? children,
   }) : assert(childManager != null),
        assert(offset != null),
        assert(diameterRatio != null),
@@ -208,6 +207,8 @@ class RenderListWheelViewport
       'rendered outside will be clipped anyway.';
 
   /// The delegate that manages the children of this object.
+  ///
+  /// This delegate must maintain the [ListWheelParentData.index] value.
   final ListWheelChildManager childManager;
 
   /// The associated ViewportOffset object for the viewport describing the part
@@ -530,7 +531,7 @@ class RenderListWheelViewport
     if (childManager.childCount == null)
       return double.infinity;
 
-    return math.max(0.0, (childManager.childCount - 1) * _itemExtent);
+    return math.max(0.0, (childManager.childCount! - 1) * _itemExtent);
   }
 
   /// Scroll extent distance in the untransformed plane between the center
@@ -567,7 +568,7 @@ class RenderListWheelViewport
 
   double _getIntrinsicCrossAxis(_ChildSizingFunction childSize) {
     double extent = 0.0;
-    RenderBox child = firstChild;
+    RenderBox? child = firstChild;
     while (child != null) {
       extent = math.max(extent, childSize(child));
       child = childAfter(child);
@@ -593,14 +594,14 @@ class RenderListWheelViewport
   double computeMinIntrinsicHeight(double width) {
     if (childManager.childCount == null)
       return 0.0;
-    return childManager.childCount * _itemExtent;
+    return childManager.childCount! * _itemExtent;
   }
 
   @override
   double computeMaxIntrinsicHeight(double width) {
     if (childManager.childCount == null)
       return 0.0;
-    return childManager.childCount * _itemExtent;
+    return childManager.childCount! * _itemExtent;
   }
 
   @override
@@ -611,12 +612,14 @@ class RenderListWheelViewport
     size = constraints.biggest;
   }
 
-  /// Gets the index of a child by looking at its parentData.
+  /// Gets the index of a child by looking at its [parentData].
+  ///
+  /// This relies on the [childManager] maintaining [ListWheelParentData.index].
   int indexOf(RenderBox child) {
     assert(child != null);
     final ListWheelParentData childParentData = child.parentData as ListWheelParentData;
     assert(childParentData.index != null);
-    return childParentData.index;
+    return childParentData.index!;
   }
 
   /// Returns the index of the child at the given offset.
@@ -625,7 +628,7 @@ class RenderListWheelViewport
   /// Returns the scroll offset of the child with the given index.
   double indexToScrollOffset(int index) => index * itemExtent;
 
-  void _createChild(int index, { RenderBox after }) {
+  void _createChild(int index, { RenderBox? after }) {
     invokeLayoutCallback<BoxConstraints>((BoxConstraints constraints) {
       assert(constraints == this.constraints);
       childManager.createChild(index, after: after);
@@ -697,7 +700,7 @@ class RenderListWheelViewport
     // return.
     if (targetFirstIndex > targetLastIndex) {
       while (firstChild != null)
-        _destroyChild(firstChild);
+        _destroyChild(firstChild!);
       return;
     }
 
@@ -711,34 +714,34 @@ class RenderListWheelViewport
 
     // Case when there is no intersection.
     if (childCount > 0 &&
-        (indexOf(firstChild) > targetLastIndex || indexOf(lastChild) < targetFirstIndex)) {
+        (indexOf(firstChild!) > targetLastIndex || indexOf(lastChild!) < targetFirstIndex)) {
       while (firstChild != null)
-        _destroyChild(firstChild);
+        _destroyChild(firstChild!);
     }
 
     // If there is no child at this stage, we add the first one that is in
     // target range.
     if (childCount == 0) {
       _createChild(targetFirstIndex);
-      _layoutChild(firstChild, childConstraints, targetFirstIndex);
+      _layoutChild(firstChild!, childConstraints, targetFirstIndex);
     }
 
-    int currentFirstIndex = indexOf(firstChild);
-    int currentLastIndex = indexOf(lastChild);
+    int currentFirstIndex = indexOf(firstChild!);
+    int currentLastIndex = indexOf(lastChild!);
 
     // Remove all unnecessary children by shortening the current child list, in
     // both directions.
     while (currentFirstIndex < targetFirstIndex) {
-      _destroyChild(firstChild);
+      _destroyChild(firstChild!);
       currentFirstIndex++;
     }
     while (currentLastIndex > targetLastIndex) {
-      _destroyChild(lastChild);
+      _destroyChild(lastChild!);
       currentLastIndex--;
     }
 
     // Relayout all active children.
-    RenderBox child = firstChild;
+    RenderBox? child = firstChild;
     while (child != null) {
       child.layout(childConstraints, parentUsesSize: true);
       child = childAfter(child);
@@ -747,11 +750,11 @@ class RenderListWheelViewport
     // Spawning new children that are actually visible but not in child list yet.
     while (currentFirstIndex > targetFirstIndex) {
       _createChild(currentFirstIndex - 1);
-      _layoutChild(firstChild, childConstraints, --currentFirstIndex);
+      _layoutChild(firstChild!, childConstraints, --currentFirstIndex);
     }
     while (currentLastIndex < targetLastIndex) {
       _createChild(currentLastIndex + 1, after: lastChild);
-      _layoutChild(lastChild, childConstraints, ++currentLastIndex);
+      _layoutChild(lastChild!, childConstraints, ++currentLastIndex);
     }
 
     offset.applyViewportDimension(_viewportExtent);
@@ -796,13 +799,11 @@ class RenderListWheelViewport
 
   /// Paints all children visible in the current viewport.
   void _paintVisibleChildren(PaintingContext context, Offset offset) {
-    RenderBox childToPaint = firstChild;
-    ListWheelParentData childParentData = childToPaint?.parentData as ListWheelParentData;
-
-    while (childParentData != null) {
+    RenderBox? childToPaint = firstChild;
+    while (childToPaint != null) {
+      final ListWheelParentData childParentData = childToPaint.parentData! as ListWheelParentData;
       _paintTransformedChild(childToPaint, context, offset, childParentData.offset);
       childToPaint = childAfter(childToPaint);
-      childParentData = childToPaint?.parentData as ListWheelParentData;
     }
   }
 
@@ -988,23 +989,23 @@ class RenderListWheelViewport
   /// painting coordinates** system.
   @override
   void applyPaintTransform(RenderBox child, Matrix4 transform) {
-    final ListWheelParentData parentData = child?.parentData as ListWheelParentData;
+    final ListWheelParentData parentData = child.parentData as ListWheelParentData;
     transform.translate(0.0, _getUntransformedPaintingCoordinateY(parentData.offset.dy));
   }
 
   @override
-  Rect describeApproximatePaintClip(RenderObject child) {
-    if (child != null && _shouldClipAtCurrentOffset()) {
+  Rect? describeApproximatePaintClip(RenderObject child) {
+    if (_shouldClipAtCurrentOffset()) {
       return Offset.zero & size;
     }
     return null;
   }
 
   @override
-  bool hitTestChildren(BoxHitTestResult result, { Offset position }) => false;
+  bool hitTestChildren(BoxHitTestResult result, { required Offset position }) => false;
 
   @override
-  RevealedOffset getOffsetToReveal(RenderObject target, double alignment, { Rect rect }) {
+  RevealedOffset getOffsetToReveal(RenderObject target, double alignment, { Rect? rect }) {
     // `target` is only fully revealed when in the selected/center position. Therefore,
     // this method always returns the offset that shows `target` in the center position,
     // which is the same offset for all `alignment` values.
@@ -1028,8 +1029,8 @@ class RenderListWheelViewport
 
   @override
   void showOnScreen({
-    RenderObject descendant,
-    Rect rect,
+    RenderObject? descendant,
+    Rect? rect,
     Duration duration = Duration.zero,
     Curve curve = Curves.ease,
   }) {

packages/flutter/lib/src/rendering/performance_overlay.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 'box.dart';
 import 'layer.dart';
 import 'object.dart';

packages/flutter/lib/src/rendering/table_border.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';
 import 'package:flutter/painting.dart' hide Border;
 
@@ -150,12 +148,12 @@ class TableBorder {
   /// borders.
   ///
   /// {@macro dart.ui.shadow.lerp}
-  static TableBorder lerp(TableBorder a, TableBorder b, double t) {
+  static TableBorder? lerp(TableBorder? a, TableBorder? b, double t) {
     assert(t != null);
     if (a == null && b == null)
       return null;
     if (a == null)
-      return b.scale(t);
+      return b!.scale(t);
     if (b == null)
       return a.scale(1.0 - t);
     return TableBorder(
@@ -197,8 +195,8 @@ class TableBorder {
   void paint(
     Canvas canvas,
     Rect rect, {
-    @required Iterable<double> rows,
-    @required Iterable<double> columns,
+    required Iterable<double> rows,
+    required Iterable<double> columns,
   }) {
     // properties can't be null
     assert(top != null);

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

@@ -216,8 +216,8 @@ mixin ServicesBinding on BindingBase, SchedulerBinding {
   /// To use a different [RestorationManager] subclasses can override
   /// [createRestorationManager], which is called to create the instance
   /// returned by this getter.
-  RestorationManager get restorationManager => _restorationManager!;
-  RestorationManager? _restorationManager;
+  RestorationManager get restorationManager => _restorationManager;
+  late RestorationManager _restorationManager;
 
   /// Creates the [RestorationManager] instance available via
   /// [restorationManager].

packages/flutter/lib/src/widgets/framework.dart

@@ -2677,8 +2677,12 @@ class BuildOwner {
             e,
             stack,
             informationCollector: () sync* {
+              if (index < _dirtyElements.length) {
                 yield DiagnosticsDebugCreator(DebugCreator(_dirtyElements[index]));
                 yield _dirtyElements[index].describeElement('The element being rebuilt at the time was index $index of $dirtyCount');
+              } else {
+                yield ErrorHint('The element being rebuilt at the time was index $index of $dirtyCount, but _dirtyElements only had ${_dirtyElements.length} entries. This suggests some confusion in the framework internals.');
+              }
             },
           );
         }