Remove 'must not be null' comments from painting and rendering libraries. (flutter#134993)

## 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 painting and rendering libraries.

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after null safety has been made the default, these comments were largely fragile, in that it was easy for them to get out of date, and not be accurate anymore anyhow.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#134991
- flutter#134992
- flutter#134994

## Tests
 - Documentation only change.

Author: gspencergoog

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

@@ -20,8 +20,6 @@ typedef _SimpleDecoderCallback = Future<ui.Codec> Function(ui.ImmutableBuffer bu
 @immutable
 class NetworkImage extends image_provider.ImageProvider<image_provider.NetworkImage> implements image_provider.NetworkImage {
   /// Creates an object that fetches the image at the given URL.
-  ///
-  /// The arguments [url] and [scale] must not be null.
   const NetworkImage(this.url, { this.scale = 1.0, this.headers });
 
   @override

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

@@ -40,8 +40,6 @@ class NetworkImage
     extends image_provider.ImageProvider<image_provider.NetworkImage>
     implements image_provider.NetworkImage {
   /// Creates an object that fetches the image at the given URL.
-  ///
-  /// The arguments [url] and [scale] must not be null.
   const NetworkImage(this.url, {this.scale = 1.0, this.headers});
 
   @override

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

@@ -185,8 +185,6 @@ abstract class AlignmentGeometry {
 ///    whether the horizontal direction depends on the [TextDirection].
 class Alignment extends AlignmentGeometry {
   /// Creates an alignment.
-  ///
-  /// The [x] and [y] arguments must not be null.
   const Alignment(this.x, this.y);
 
   /// The distance fraction in the horizontal direction.
@@ -401,8 +399,6 @@ class Alignment extends AlignmentGeometry {
 ///    whose horizontal component does not depend on the text direction).
 class AlignmentDirectional extends AlignmentGeometry {
   /// Creates a directional alignment.
-  ///
-  /// The [start] and [y] arguments must not be null.
   const AlignmentDirectional(this.start, this.y);
 
   /// The distance fraction in the horizontal direction.

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

@@ -20,8 +20,6 @@ import 'borders.dart';
 class BeveledRectangleBorder extends OutlinedBorder {
   /// Creates a border like a [RoundedRectangleBorder] except that the corners
   /// are joined by straight lines instead of arcs.
-  ///
-  /// The arguments must not be null.
   const BeveledRectangleBorder({
     super.side,
     this.borderRadius = BorderRadius.zero,

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

@@ -77,8 +77,6 @@ class BorderSide with Diagnosticable {
   /// If one of the sides is zero-width with [BorderStyle.none], then the other
   /// side is return as-is. If both of the sides are zero-width with
   /// [BorderStyle.none], then [BorderSide.none] is returned.
-  ///
-  /// The arguments must not be null.
   static BorderSide merge(BorderSide a, BorderSide b) {
     assert(canMerge(a, b));
     final bool aIsNone = a.style == BorderStyle.none && a.width == 0.0;
@@ -243,8 +241,6 @@ class BorderSide with Diagnosticable {
   ///
   /// Two sides can be merged if one or both are zero-width with
   /// [BorderStyle.none], or if they both have the same color and style.
-  ///
-  /// The arguments must not be null.
   static bool canMerge(BorderSide a, BorderSide b) {
     if ((a.style == BorderStyle.none && a.width == 0.0) ||
         (b.style == BorderStyle.none && b.width == 0.0)) {
@@ -256,8 +252,6 @@ class BorderSide with Diagnosticable {
 
   /// Linearly interpolate between two border sides.
   ///
-  /// The arguments must not be null.
-  ///
   /// {@macro dart.ui.shadow.lerp}
   static BorderSide lerp(BorderSide a, BorderSide b, double t) {
     if (identical(a, b)) {
@@ -665,8 +659,6 @@ abstract class ShapeBorder {
 abstract class OutlinedBorder extends ShapeBorder {
   /// Abstract const constructor. This constructor enables subclasses to provide
   /// const constructors so that they can be used in const expressions.
-  ///
-  /// The value of [side] must not be null.
   const OutlinedBorder({ this.side = BorderSide.none });
 
   @override
@@ -884,8 +876,6 @@ class _CompoundBorder extends ShapeBorder {
 /// borders (where all the borders have the same configuration); to render a
 /// uniform border, consider using [Canvas.drawRect] directly.
 ///
-/// The arguments must not be null.
-///
 /// See also:
 ///
 ///  * [paintImage], which paints an image in a rectangle on a canvas.

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

@@ -387,8 +387,6 @@ class Border extends BoxBorder {
   /// Creates a border.
   ///
   /// All the sides of the border default to [BorderSide.none].
-  ///
-  /// The arguments must not be null.
   const Border({
     this.top = BorderSide.none,
     this.right = BorderSide.none,
@@ -397,8 +395,6 @@ class Border extends BoxBorder {
   });
 
   /// Creates a border whose sides are all the same.
-  ///
-  /// The `side` argument must not be null.
   const Border.fromBorderSide(BorderSide side)
       : top = side,
         right = side,
@@ -410,7 +406,7 @@ class Border extends BoxBorder {
   /// The `vertical` argument applies to the [left] and [right] sides, and the
   /// `horizontal` argument applies to the [top] and [bottom] sides.
   ///
-  /// All arguments default to [BorderSide.none] and must not be null.
+  /// All arguments default to [BorderSide.none].
   const Border.symmetric({
     BorderSide vertical = BorderSide.none,
     BorderSide horizontal = BorderSide.none,
@@ -437,8 +433,6 @@ class Border extends BoxBorder {
   ///
   /// It is only valid to call this if [BorderSide.canMerge] returns true for
   /// the pairwise combination of each side on both [Border]s.
-  ///
-  /// The arguments must not be null.
   static Border merge(Border a, Border b) {
     assert(BorderSide.canMerge(a.top, b.top));
     assert(BorderSide.canMerge(a.right, b.right));
@@ -761,8 +755,6 @@ class BorderDirectional extends BoxBorder {
   /// the trailing edge. They are resolved during [paint].
   ///
   /// All the sides of the border default to [BorderSide.none].
-  ///
-  /// The arguments must not be null.
   const BorderDirectional({
     this.top = BorderSide.none,
     this.start = BorderSide.none,
@@ -775,8 +767,6 @@ class BorderDirectional extends BoxBorder {
   ///
   /// It is only valid to call this if [BorderSide.canMerge] returns true for
   /// the pairwise combination of each side on both [BorderDirectional]s.
-  ///
-  /// The arguments must not be null.
   static BorderDirectional merge(BorderDirectional a, BorderDirectional b) {
     assert(BorderSide.canMerge(a.top, b.top));
     assert(BorderSide.canMerge(a.start, b.start));

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

@@ -84,8 +84,6 @@ class BoxDecoration extends Decoration {
   /// * If [boxShadow] is null, this decoration does not paint a shadow.
   /// * If [gradient] is null, this decoration does not paint gradients.
   /// * If [backgroundBlendMode] is null, this decoration paints with [BlendMode.srcOver]
-  ///
-  /// The [shape] argument must not be null.
   const BoxDecoration({
     this.color,
     this.image,

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

@@ -30,8 +30,6 @@ import 'borders.dart';
 ///  * [Border], which, when used with [BoxDecoration], can also describe a circle.
 class CircleBorder extends OutlinedBorder {
   /// Create a circle border.
-  ///
-  /// The [side] argument must not be null.
   const CircleBorder({ super.side, this.eccentricity = 0.0 })
       : assert(eccentricity >= 0.0, 'The eccentricity argument $eccentricity is not greater than or equal to zero.'),
         assert(eccentricity <= 1.0, 'The eccentricity argument $eccentricity is not less than or equal to one.');

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

@@ -86,8 +86,8 @@ Color _colorFromHue(
 class HSVColor {
   /// Creates a color.
   ///
-  /// All the arguments must not be null and be in their respective ranges. See
-  /// the fields for each parameter for a description of their ranges.
+  /// All the arguments must be in their respective ranges. See the fields for
+  /// each parameter for a description of their ranges.
   const HSVColor.fromAHSV(this.alpha, this.hue, this.saturation, this.value)
     : assert(alpha >= 0.0),
       assert(alpha <= 1.0),
@@ -254,8 +254,8 @@ class HSVColor {
 class HSLColor {
   /// Creates a color.
   ///
-  /// All the arguments must not be null and be in their respective ranges. See
-  /// the fields for each parameter for a description of their ranges.
+  /// All the arguments must be in their respective ranges. See the fields for
+  /// each parameter for a description of their ranges.
   const HSLColor.fromAHSL(this.alpha, this.hue, this.saturation, this.lightness)
     : assert(alpha >= 0.0),
       assert(alpha <= 1.0),
@@ -499,8 +499,6 @@ class ColorSwatch<T> extends Color {
 /// [DiagnosticsProperty] that has an [Color] as value.
 class ColorProperty extends DiagnosticsProperty<Color> {
   /// Create a diagnostics property for [Color].
-  ///
-  /// The [showName], [style], and [level] arguments must not be null.
   ColorProperty(
     String super.name,
     super.value, {

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

@@ -33,7 +33,7 @@ import 'edge_insets.dart';
 ///    radius in a step function instead of gradually like the
 ///    [ContinuousRectangleBorder].
 class ContinuousRectangleBorder extends OutlinedBorder {
-  /// The arguments must not be null.
+  /// Creates a [ContinuousRectangleBorder].
   const ContinuousRectangleBorder({
     super.side,
     this.borderRadius = BorderRadius.zero,