Use int for PlaceholderSpan.placeholderCodeUnit (#98971)

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

@@ -42,7 +42,7 @@ abstract class PlaceholderSpan extends InlineSpan {
   }) : super(style: style);
 
   /// The unicode character to represent a placeholder.
-  static const String placeholderCodeUnit = '\uFFFC';
+  static const int placeholderCodeUnit = 0xFFFC;
 
   /// How the placeholder aligns vertically with the text.
   ///
@@ -60,7 +60,7 @@ abstract class PlaceholderSpan extends InlineSpan {
   @override
   void computeToPlainText(StringBuffer buffer, {bool includeSemanticsLabels = true, bool includePlaceholders = true}) {
     if (includePlaceholders) {
-      buffer.write(placeholderCodeUnit);
+      buffer.writeCharCode(placeholderCodeUnit);
     }
   }
 

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

@@ -140,7 +140,7 @@ class WidgetSpan extends PlaceholderSpan {
   @override
   int? codeUnitAtVisitor(int index, Accumulator offset) {
     offset.increment(1);
-    return PlaceholderSpan.placeholderCodeUnit.codeUnitAt(0);
+    return PlaceholderSpan.placeholderCodeUnit;
   }
 
   @override