Remove 'must be non-null' and 'must not be null' comments in widgets …

…library (#134992)

## Description

This removes all of the comments that are of the form "so-and-so (must not be null|can ?not be null|must be non-null)" from the cases where those values are defines as non-nullable values.

This PR removes them from the widgets library.

This was done by hand, since it really didn't lend itself to scripting, so it needs to be more than just spot-checked, I think. I was careful to leave any comment that referred to parameters that were nullable, but I may have missed some.

In addition to being no longer relevant after null safety has been made the default, these comments were largely fragile, in that it was easy for them to get out of date, and not be accurate anymore anyhow.

This did create a number of constructor comments which basically say "Creates a [Foo].", but I don't really know how to avoid that in a large scale change, since there's not much you can really say in a lot of cases.  I think we might consider some leniency for constructors to the "Comment must be meaningful" style guidance (which we de facto have already, since there are a bunch of these).

## Related PRs
- flutter/flutter#134984
- flutter/flutter#134991
- flutter/flutter#134993
- flutter/flutter#134994

## Tests
 - Documentation only change.

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

@@ -455,8 +455,6 @@ abstract class Action<T extends Intent> with Diagnosticable {
 @immutable
 class ActionListener extends StatefulWidget {
   /// Create a const [ActionListener].
-  ///
-  /// The [listener], [action], and [child] arguments must not be null.
   const ActionListener({
     super.key,
     required this.listener,
@@ -465,13 +463,9 @@ class ActionListener extends StatefulWidget {
   });
 
   /// The [ActionListenerCallback] callback to register with the [action].
-  ///
-  /// Must not be null.
   final ActionListenerCallback listener;
 
   /// The [Action] that the callback will be registered with.
-  ///
-  /// Must not be null.
   final Action<Intent> action;
 
   /// {@macro flutter.widgets.ProxyWidget.child}
@@ -712,8 +706,6 @@ class ActionDispatcher with Diagnosticable {
 ///  * [ActionDispatcher], the object that this widget uses to manage actions.
 class Actions extends StatefulWidget {
   /// Creates an [Actions] widget.
-  ///
-  /// The [child], [actions], and [dispatcher] arguments must not be null.
   const Actions({
     super.key,
     this.dispatcher,
@@ -950,8 +942,6 @@ class Actions extends StatefulWidget {
   /// This method returns the result of invoking the action's [Action.invoke]
   /// method.
   ///
-  /// The `context` and `intent` arguments must not be null.
-  ///
   /// If the given `intent` doesn't map to an action, then it will look to the
   /// next ancestor [Actions] widget in the hierarchy until it reaches the root.
   ///
@@ -1003,8 +993,6 @@ class Actions extends StatefulWidget {
   /// first action found was disabled, or the action itself returns null
   /// from [Action.invoke], then this method returns null.
   ///
-  /// The `context` and `intent` arguments must not be null.
-  ///
   /// If the given `intent` doesn't map to an action, then it will look to the
   /// next ancestor [Actions] widget in the hierarchy until it reaches the root.
   /// If a suitable [Action] is found but its [Action.isEnabled] returns false,
@@ -1153,8 +1141,6 @@ class _ActionsScope extends InheritedWidget {
 /// It hosts its own [FocusNode] or uses [focusNode], if given.
 class FocusableActionDetector extends StatefulWidget {
   /// Create a const [FocusableActionDetector].
-  ///
-  /// The [enabled], [autofocus], [mouseCursor], and [child] arguments must not be null.
   const FocusableActionDetector({
     super.key,
     this.enabled = true,

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

@@ -115,8 +115,6 @@ class AnimatedCrossFade extends StatefulWidget {
   /// The [duration] of the animation is the same for all components (fade in,
   /// fade out, and size), and you can pass [Interval]s instead of [Curve]s in
   /// order to have finer control, e.g., creating an overlap between the fades.
-  ///
-  /// All the arguments other than [key] must be non-null.
   const AnimatedCrossFade({
     super.key,
     required this.firstChild,

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

@@ -23,8 +23,6 @@ import 'ticker_provider.dart';
 ///  * [SizeTransition], which changes its size based on an [Animation].
 class AnimatedSize extends StatefulWidget {
   /// Creates a widget that animates its size to match that of its child.
-  ///
-  /// The [curve] and [duration] arguments must not be null.
   const AnimatedSize({
     super.key,
     this.child,
@@ -77,7 +75,7 @@ class AnimatedSize extends StatefulWidget {
 
   /// {@macro flutter.material.Material.clipBehavior}
   ///
-  /// Defaults to [Clip.hardEdge], and must not be null.
+  /// Defaults to [Clip.hardEdge].
   final Clip clipBehavior;
 
   @override

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

@@ -103,9 +103,6 @@ typedef AnimatedSwitcherLayoutBuilder = Widget Function(Widget? currentChild, Li
 ///  * [FadeTransition], which [AnimatedSwitcher] uses to perform the transition.
 class AnimatedSwitcher extends StatefulWidget {
   /// Creates an [AnimatedSwitcher].
-  ///
-  /// The [duration], [transitionBuilder], [layoutBuilder], [switchInCurve], and
-  /// [switchOutCurve] parameters must not be null.
   const AnimatedSwitcher({
     super.key,
     this.child,

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

@@ -264,8 +264,6 @@ class WidgetsApp extends StatefulWidget {
   /// Creates a widget that wraps a number of widgets that are commonly
   /// required for an application.
   ///
-  /// The boolean arguments, [color], and [navigatorObservers] must not be null.
-  ///
   /// Most callers will want to use the [home] or [routes] parameters, or both.
   /// The [home] parameter is a convenience for the following [routes] map:
   ///

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

@@ -386,8 +386,6 @@ class StreamBuilder<T> extends StreamBuilderBase<T, AsyncSnapshot<T>> {
   /// strategy is given by [builder].
   ///
   /// The [initialData] is used to create the initial snapshot.
-  ///
-  /// The [builder] must not be null.
   const StreamBuilder({
     super.key,
     this.initialData,
@@ -517,8 +515,6 @@ class StreamBuilder<T> extends StreamBuilderBase<T, AsyncSnapshot<T>> {
 class FutureBuilder<T> extends StatefulWidget {
   /// Creates a widget that builds itself based on the latest snapshot of
   /// interaction with a [Future].
-  ///
-  /// The [builder] must not be null.
   const FutureBuilder({
     super.key,
     required this.future,

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

@@ -64,8 +64,6 @@ enum AutofillContextAction {
 ///   clean up actions to be run when a topmost [AutofillGroup] is disposed.
 class AutofillGroup extends StatefulWidget {
   /// Creates a scope for autofillable input fields.
-  ///
-  /// The [child] argument must not be null.
   const AutofillGroup({
     super.key,
     required this.child,

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

@@ -294,8 +294,6 @@ class _AutomaticKeepAliveState extends State<AutomaticKeepAlive> {
 /// [KeepAliveNotification] internally.
 class KeepAliveNotification extends Notification {
   /// Creates a notification to indicate that a subtree must be kept alive.
-  ///
-  /// The [handle] must not be null.
   const KeepAliveNotification(this.handle);
 
   /// A [Listenable] that will inform its clients when the widget that fired the

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

@@ -54,9 +54,6 @@ enum BannerLocation {
 /// Paints a [Banner].
 class BannerPainter extends CustomPainter {
   /// Creates a banner painter.
-  ///
-  /// The [message], [textDirection], [location], and [layoutDirection]
-  /// arguments must not be null.
   BannerPainter({
     required this.message,
     required this.textDirection,
@@ -236,8 +233,6 @@ class BannerPainter extends CustomPainter {
 ///    debug mode, to show a banner that says "DEBUG".
 class Banner extends StatelessWidget {
   /// Creates a banner.
-  ///
-  /// The [message] and [location] arguments must not be null.
   const Banner({
     super.key,
     this.child,