Remove Placeholder. It has no clients. (#9563)

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

-// Copyright 2015 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-import 'basic.dart';
-import 'framework.dart';
-
-/// A widget whose child can be mutated.
-class Placeholder extends StatefulWidget {
-  /// Creates a widget whose child can be mutated.
-  const Placeholder({ Key key }) : super(key: key);
-
-  @override
-  PlaceholderState createState() => new PlaceholderState();
-}
-
-/// State for a [Placeholder] widget.
-///
-/// Useful for setting the child currently displayed by this placeholder widget.
-class PlaceholderState extends State<Placeholder> {
-  /// The child that this widget builds.
-  ///
-  /// Mutating this field will cause this widget to rebuild with the new child.
-  Widget get child => _child;
-  Widget _child;
-  set child(Widget value) {
-    if (_child == value)
-      return;
-    setState(() {
-      _child = value;
-    });
-  }
-
-  @override
-  Widget build(BuildContext context) {
-    if (_child != null)
-      return child;
-    return const SizedBox(width: 0.0, height: 0.0);
-  }
-}

packages/flutter/lib/widgets.dart

@@ -43,7 +43,6 @@ export 'src/widgets/page_storage.dart';
 export 'src/widgets/page_view.dart';
 export 'src/widgets/pages.dart';
 export 'src/widgets/performance_overlay.dart';
-export 'src/widgets/placeholder.dart';
 export 'src/widgets/preferred_size.dart';
 export 'src/widgets/primary_scroll_controller.dart';
 export 'src/widgets/raw_keyboard_listener.dart';

packages/flutter/test/widgets/framework_test.dart

@@ -11,6 +11,11 @@ class TestState extends State<StatefulWidget> {
   Widget build(BuildContext context) => null;
 }
 
+// TODO(ianh): Remove this once we add the real Placeholder widget
+class Placeholder extends Container {
+  Placeholder({ Key key }) : super(key: key);
+}
+
 void main() {
   testWidgets('UniqueKey control test', (WidgetTester tester) async {
     final Key key = new UniqueKey();
@@ -440,7 +445,7 @@ void main() {
     final GlobalKey key = new GlobalKey();
     await tester.pumpWidget(new Container(key: key));
     expect(log, isEmpty);
-    await tester.pumpWidget(const Placeholder());
+    await tester.pumpWidget(new Placeholder());
     debugPrint = oldCallback;
     debugPrintGlobalKeyedWidgetLifecycle = false;
 

packages/flutter/test/widgets/placeholder_test.dart

-// Copyright 2017 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-import 'package:flutter_test/flutter_test.dart';
-import 'package:flutter/widgets.dart';
-
-void main() {
-  testWidgets('Placeholder control test', (WidgetTester tester) async {
-    final GlobalKey<PlaceholderState> key = new GlobalKey<PlaceholderState>();
-
-    await tester.pumpWidget(new Placeholder(key: key));
-
-    key.currentState.child = const Text('target');
-
-    expect(find.text('target'), findsNothing);
-
-    await tester.pump();
-
-    expect(find.text('target'), findsOneWidget);
-  });
-}