Fix text direction nullability (#111513)

Author: dnfield

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

@@ -222,11 +222,13 @@ class TextPainter {
   /// This is a convenience method that creates a text painter with the supplied
   /// parameters, lays it out with the supplied [minWidth] and [maxWidth], and
   /// returns its [TextPainter.width] making sure to dispose the underlying
-  /// resources.
+  /// resources. Doing this operation is expensive and should be avoided
+  /// whenever it is possible to preserve the [TextPainter] to paint the
+  /// text or get other information about it.
   static double computeWidth({
     required InlineSpan text,
+    required TextDirection textDirection,
     TextAlign textAlign = TextAlign.start,
-    TextDirection? textDirection,
     double textScaleFactor = 1.0,
     int? maxLines,
     String? ellipsis,
@@ -262,11 +264,13 @@ class TextPainter {
   /// This is a convenience method that creates a text painter with the supplied
   /// parameters, lays it out with the supplied [minWidth] and [maxWidth], and
   /// returns its [TextPainter.maxIntrinsicWidth] making sure to dispose the
-  /// underlying resources.
+  /// underlying resources. Doing this operation is expensive and should be avoided
+  /// whenever it is possible to preserve the [TextPainter] to paint the
+  /// text or get other information about it.
   static double computeMaxIntrinsicWidth({
     required InlineSpan text,
+    required TextDirection textDirection,
     TextAlign textAlign = TextAlign.start,
-    TextDirection? textDirection,
     double textScaleFactor = 1.0,
     int? maxLines,
     String? ellipsis,