Remove 'must be non-null' and 'must not be null' comments from material. (flutter#134991)

## 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 material 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after null safety has been made the default, these comments were largely fragile, in that it was easy for them to get out of date, and not be accurate anymore anyhow.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#134984
- flutter#134992
- flutter#134993
- flutter#134994

## Tests
 - Documentation only change.

Author: gspencergoog

packages/flutter/lib/src/material/animated_icons/animated_icons.dart

@@ -29,8 +29,8 @@ part of material_animated_icons; // ignore: use_string_in_part_of_directives
 class AnimatedIcon extends StatelessWidget {
   /// Creates an AnimatedIcon.
   ///
-  /// The [progress] and [icon] arguments must not be null.
-  /// The [size] and [color] default to the value given by the current [IconTheme].
+  /// The [size] and [color] default to the value given by the current
+  /// [IconTheme].
   const AnimatedIcon({
     super.key,
     required this.icon,

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

@@ -202,8 +202,6 @@ class MaterialApp extends StatefulWidget {
   /// unsupported route.
   ///
   /// This class creates an instance of [WidgetsApp].
-  ///
-  /// The boolean arguments, [routes], and [navigatorObservers], must not be null.
   const MaterialApp({
     super.key,
     this.navigatorKey,

packages/flutter/lib/src/material/app_bar.dart

@@ -1436,9 +1436,6 @@ class _SliverAppBarDelegate extends SliverPersistentHeaderDelegate {
 ///  * <https://material.io/design/components/app-bars-top.html>
 class SliverAppBar extends StatefulWidget {
   /// Creates a Material Design app bar that can be placed in a [CustomScrollView].
-  ///
-  /// The arguments [forceElevated], [primary], [floating], [pinned], [snap]
-  /// and [automaticallyImplyLeading] must not be null.
   const SliverAppBar({
     super.key,
     this.leading,

packages/flutter/lib/src/material/app_bar_theme.dart

@@ -200,8 +200,6 @@ class AppBarTheme with Diagnosticable {
 
   /// Linearly interpolate between two AppBar themes.
   ///
-  /// The argument `t` must not be null.
-  ///
   /// {@macro dart.ui.shadow.lerp}
   static AppBarTheme lerp(AppBarTheme? a, AppBarTheme? b, double t) {
     if (identical(a, b) && a != null) {

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

@@ -93,9 +93,8 @@ enum MaterialBannerClosedReason {
 class MaterialBanner extends StatefulWidget {
   /// Creates a [MaterialBanner].
   ///
-  /// The [actions], [content], and [forceActionsBelow] must be non-null.
-  /// The [actions].length must be greater than 0. The [elevation] must be null or
-  /// non-negative.
+  /// The length of the [actions] list must not be empty. The [elevation] must
+  /// be null or non-negative.
   const MaterialBanner({
     super.key,
     required this.content,

packages/flutter/lib/src/material/banner_theme.dart

@@ -97,8 +97,6 @@ class MaterialBannerThemeData with Diagnosticable {
 
   /// Linearly interpolate between two Banner themes.
   ///
-  /// The argument `t` must not be null.
-  ///
   /// {@macro dart.ui.shadow.lerp}
   static MaterialBannerThemeData lerp(MaterialBannerThemeData? a, MaterialBannerThemeData? b, double t) {
     return MaterialBannerThemeData(

packages/flutter/lib/src/material/bottom_app_bar_theme.dart

@@ -94,8 +94,6 @@ class BottomAppBarTheme with Diagnosticable {
 
   /// Linearly interpolate between two BAB themes.
   ///
-  /// The argument `t` must not be null.
-  ///
   /// {@macro dart.ui.shadow.lerp}
   static BottomAppBarTheme lerp(BottomAppBarTheme? a, BottomAppBarTheme? b, double t) {
     if (identical(a, b) && a != null) {

packages/flutter/lib/src/material/bottom_navigation_bar.dart

@@ -80,8 +80,8 @@ enum BottomNavigationBarLandscapeLayout {
 /// [BottomNavigationBarType.fixed] when there are less than four items, and
 /// [BottomNavigationBarType.shifting] otherwise.
 ///
-/// The length of [items] must be at least two and each item's icon and title/label
-/// must not be null.
+/// The length of [items] must be at least two and each item's icon and
+/// label must not be null.
 ///
 ///  * [BottomNavigationBarType.fixed], the default when there are less than
 ///    four [items]. The selected item is rendered with the
@@ -188,7 +188,7 @@ class BottomNavigationBar extends StatefulWidget {
   /// are two or three [items], [BottomNavigationBarType.shifting] otherwise.
   ///
   /// The [iconSize], [selectedFontSize], [unselectedFontSize], and [elevation]
-  /// arguments must be non-null and non-negative.
+  /// arguments must be non-negative.
   ///
   /// If [selectedLabelStyle].color and [unselectedLabelStyle].color values
   /// are non-null, they will be used instead of [selectedItemColor] and

packages/flutter/lib/src/material/bottom_navigation_bar_theme.dart

@@ -170,8 +170,6 @@ class BottomNavigationBarThemeData with Diagnosticable {
 
   /// Linearly interpolate between two [BottomNavigationBarThemeData].
   ///
-  /// The argument `t` must not be null.
-  ///
   /// {@macro dart.ui.shadow.lerp}
   static BottomNavigationBarThemeData lerp(BottomNavigationBarThemeData? a, BottomNavigationBarThemeData? b, double t) {
     if (identical(a, b) && a != null) {
@@ -276,8 +274,6 @@ class BottomNavigationBarThemeData with Diagnosticable {
 class BottomNavigationBarTheme extends InheritedWidget {
   /// Constructs a bottom navigation bar theme that configures all descendant
   /// [BottomNavigationBar] widgets.
-  ///
-  /// The [data] must not be null.
   const BottomNavigationBarTheme({
     super.key,
     required this.data,

packages/flutter/lib/src/material/bottom_sheet.dart

@@ -1133,8 +1133,6 @@ class ModalBottomSheetRoute<T> extends PopupRoute<T> {
 /// curve specified with the [curve] argument, after the finger is released. In
 /// such a case, the value of [startingPoint] would be the progress of the
 /// animation at the time when the finger was released.
-///
-/// The [startingPoint] and [curve] arguments must not be null.
 class _BottomSheetSuspendedCurve extends ParametricCurve<double> {
   /// Creates a suspended curve.
   const _BottomSheetSuspendedCurve(