Remove deprecated FlatButton (flutter#98545)

Author: Piinks

dev/integration_tests/ios_host_app/flutterapp/lib/main

@@ -166,7 +166,7 @@ class _MyHomePageState extends State<MyHomePage> {
                     style: const TextStyle(fontSize: 17.0),
                   ),
                 ),
-                const FlatButton(
+                const TextButton(
                   child: Text('POP'),
                   onPressed: SystemNavigator.pop,
                 ),

dev/integration_tests/ios_host_app/flutterapp/lib/marquee

@@ -65,7 +65,7 @@ class MarqueeState extends State<Marquee> with SingleTickerProviderStateMixin {
               child: _MarqueeText(animation: animation),
               alignment: Alignment.centerLeft,
             ),
-            const FlatButton(child: Text('POP'), onPressed: SystemNavigator.pop),
+            const TextButton(child: Text('POP'), onPressed: SystemNavigator.pop),
           ],
         ),
       ),

packages/flutter/lib/material.dart

@@ -74,7 +74,6 @@ export 'src/material/expansion_panel.dart';
 export 'src/material/expansion_tile.dart';
 export 'src/material/expansion_tile_theme.dart';
 export 'src/material/feedback.dart';
-export 'src/material/flat_button.dart';
 export 'src/material/flexible_space_bar.dart';
 export 'src/material/floating_action_button.dart';
 export 'src/material/floating_action_button_location.dart';

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

@@ -20,22 +20,20 @@ import 'theme_data.dart';
 /// Creates a button based on [Semantics], [Material], and [InkWell]
 /// widgets.
 ///
-/// ### This class is obsolete.
-///
-/// Custom button classes can be created by configuring the
-/// [ButtonStyle] of a [TextButton], [ElevatedButton] or an
-/// [OutlinedButton].
-///
-/// FlatButton has been replaced by TextButton and ButtonTheme has been
-/// replaced by TextButtonTheme. Please migrate code that uses them.
-/// There's a detailed migration guide for the new button and button
-/// theme classes in
-/// [flutter.dev/go/material-button-migration-guide](https://flutter.dev/go/material-button-migration-guide).
-///
 /// This class does not use the current [Theme] or [ButtonTheme] to
 /// compute default values for unspecified parameters. It's intended to
 /// be used for custom Material buttons that optionally incorporate defaults
 /// from the themes or from app-specific sources.
+///
+/// This class is planned to be deprecated in a future release, see
+/// [ButtonStyleButton], the base class of [TextButton], [ElevatedButton], and
+/// [OutlinedButton].
+///
+/// See also:
+///
+///  * [TextButton], a simple flat button without a shadow.
+///  * [ElevatedButton], a filled button whose material elevates when pressed.
+///  * [OutlinedButton], a [TextButton] with a border outline.
 @Category(<String>['Material', 'Button'])
 class RawMaterialButton extends StatefulWidget {
   /// Create a button based on [Semantics], [Material], and [InkWell] widgets.