Revert "`InputDecorator`: Switch hint to Opacity instead of AnimatedOpacity" (#107406)

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

@@ -2157,8 +2157,10 @@ class _InputDecoratorState extends State<InputDecorator> with TickerProviderStat
     final TextBaseline textBaseline = labelStyle.textBaseline!;
 
     final TextStyle hintStyle = _getInlineHintStyle(themeData);
-    final Widget? hint = decoration!.hintText == null ? null : Opacity(
+    final Widget? hint = decoration!.hintText == null ? null : AnimatedOpacity(
       opacity: (isEmpty && !_hasInlineLabel) ? 1.0 : 0.0,
+      duration: _kTransitionDuration,
+      curve: _kTransitionCurve,
       alwaysIncludeSemantics: true,
       child: Text(
         decoration!.hintText!,

packages/flutter/test/material/input_date_picker_form_field_test.dart

@@ -61,12 +61,13 @@ void main() {
   }
 
   double textOpacity(WidgetTester tester, String textValue) {
-    return tester.widget<Opacity>(
+    final FadeTransition opacityWidget = tester.widget<FadeTransition>(
       find.ancestor(
         of: find.text(textValue),
-        matching: find.byType(Opacity),
+        matching: find.byType(FadeTransition),
       ).first,
-    ).opacity;
+    );
+    return opacityWidget.opacity.value;
   }
 
   group('InputDatePickerFormField', () {

packages/flutter/test/material/text_field_test.dart

@@ -131,15 +131,6 @@ double getOpacity(WidgetTester tester, Finder finder) {
   ).opacity.value;
 }
 
-double getStaticOpacity(WidgetTester tester, Finder finder) {
-  return tester.widget<Opacity>(
-    find.ancestor(
-      of: finder,
-      matching: find.byType(Opacity),
-    ).first,
-  ).opacity;
-}
-
 class TestFormatter extends TextInputFormatter {
   TestFormatter(this.onFormatEditUpdate);
   FormatEditUpdateCallback onFormatEditUpdate;
@@ -3681,7 +3672,7 @@ void main() {
     // Neither the prefix or the suffix should initially be visible, only the hint.
     expect(getOpacity(tester, find.text('Prefix')), 0.0);
     expect(getOpacity(tester, find.text('Suffix')), 0.0);
-    expect(getStaticOpacity(tester, find.text('Hint')), 1.0);
+    expect(getOpacity(tester, find.text('Hint')), 1.0);
 
     await tester.tap(find.byKey(secondKey));
     await tester.pumpAndSettle();
@@ -3689,7 +3680,7 @@ void main() {
     // Focus the Input. The hint, prefix, and suffix should appear
     expect(getOpacity(tester, find.text('Prefix')), 1.0);
     expect(getOpacity(tester, find.text('Suffix')), 1.0);
-    expect(getStaticOpacity(tester, find.text('Hint')), 1.0);
+    expect(getOpacity(tester, find.text('Hint')), 1.0);
 
     // Enter some text, and the hint should disappear and the prefix and suffix
     // should continue to be visible
@@ -3698,7 +3689,7 @@ void main() {
 
     expect(getOpacity(tester, find.text('Prefix')), 1.0);
     expect(getOpacity(tester, find.text('Suffix')), 1.0);
-    expect(getStaticOpacity(tester, find.text('Hint')), 0.0);
+    expect(getOpacity(tester, find.text('Hint')), 0.0);
 
     // Check and make sure that the right styles were applied.
     final Text prefixText = tester.widget(find.text('Prefix'));