Implement persistent bottom sheet demo.

Samples/UIWidgetsGallery/gallery/demos.cs

                     documentationUrl: "https://docs.flutter.io/flutter/material/showModalBottomSheet.html",
                     buildRoute: (BuildContext context) => new ModalBottomSheetDemo()
                 ),
-//                new GalleryDemo(
-//                    title: "Bottom sheet: Persistent",
-//                    subtitle: "A bottom sheet that sticks around",
-//                    icon: GalleryIcons.bottom_sheet_persistent,
-//                    category: GalleryDemoCategory._kMaterialComponents,
-//                    routeName: PersistentBottomSheetDemo.routeName,
-//                    documentationUrl: "https://docs.flutter.io/flutter/material/ScaffoldState/showBottomSheet.html",
-//                    buildRoute: (BuildContext context) => PersistentBottomSheetDemo()
-//                ),
+                new GalleryDemo(
+                    title: "Bottom sheet: Persistent",
+                    subtitle: "A bottom sheet that sticks around",
+                    icon: GalleryIcons.bottom_sheet_persistent,
+                    category: _kMaterialComponents,
+                    routeName: PersistentBottomSheetDemo.routeName,
+                    documentationUrl: "https://docs.flutter.io/flutter/material/ScaffoldState/showBottomSheet.html",
+                    buildRoute: (BuildContext context) => new PersistentBottomSheetDemo()
+                ),
                 new GalleryDemo(
                     title: "Buttons",
                     subtitle: "Flat, raised, dropdown, and more",

Samples/UIWidgetsGallery/demo/material/persistent_bottom_sheet_demo.cs

+using System.Collections.Generic;
+using Unity.UIWidgets.material;
+using Unity.UIWidgets.painting;
+using Unity.UIWidgets.ui;
+using Unity.UIWidgets.widgets;
+using DialogUtils = Unity.UIWidgets.material.DialogUtils;
+
+namespace UIWidgetsGallery.gallery {
+    public class PersistentBottomSheetDemo : StatefulWidget {
+        public const string routeName = "/material/persistent-bottom-sheet";
+
+        public override State createState() {
+            return new _PersistentBottomSheetDemoState();
+        }
+    }
+
+    class _PersistentBottomSheetDemoState : State<PersistentBottomSheetDemo> {
+        GlobalKey<ScaffoldState> _scaffoldKey = GlobalKey<ScaffoldState>.key();
+
+        VoidCallback _showBottomSheetCallback;
+
+        public override void initState() {
+            base.initState();
+            this._showBottomSheetCallback = this._showBottomSheet;
+        }
+
+        void _showBottomSheet() {
+            this.setState(() => {
+                // disable the button
+                this._showBottomSheetCallback = null;
+            });
+            this._scaffoldKey.currentState.showBottomSheet((BuildContext context) => {
+                    ThemeData themeData = Theme.of(this.context);
+                    return new Container(
+                        decoration: new BoxDecoration(
+                            border: new Border(top: new BorderSide(color: themeData.disabledColor))
+                        ),
+                        child: new Padding(
+                            padding: EdgeInsets.all(32.0f),
+                            child: new Text("This is a Material persistent bottom sheet. Drag downwards to dismiss it.",
+                                textAlign: TextAlign.center,
+                                style: new TextStyle(
+                                    color: themeData.accentColor,
+                                    fontSize: 24.0f
+                                )
+                            )
+                        )
+                    );
+                })
+                .closed.Then((value) => {
+                    if (this.mounted) {
+                        this.setState(() => {
+                            // re-enable the button
+                            this._showBottomSheetCallback = this._showBottomSheet;
+                        });
+                    }
+                });
+        }
+
+        void _showMessage() {
+            DialogUtils.showDialog(
+                context: this.context,
+                builder: (BuildContext context) => {
+                    return new AlertDialog(
+                        content: new Text("You tapped the floating action button."),
+                        actions: new List<Widget> {
+                            new FlatButton(
+                                onPressed: () => { Navigator.pop(context); },
+                                child: new Text("OK")
+                            )
+                        }
+                    );
+                }
+            );
+        }
+
+        public override Widget build(BuildContext context) {
+            return new Scaffold(
+                key: this._scaffoldKey,
+                appBar: new AppBar(
+                    title: new Text("Persistent bottom sheet"),
+                    actions: new List<Widget> {
+                        new MaterialDemoDocumentationButton(PersistentBottomSheetDemo.routeName)
+                    }
+                ),
+                floatingActionButton: new FloatingActionButton(
+                    onPressed: this._showMessage,
+                    backgroundColor: Colors.redAccent,
+                    child: new Icon(
+                        Icons.add
+                    )
+                ),
+                body: new Center(
+                    child: new RaisedButton(
+                        onPressed: this._showBottomSheetCallback,
+                        child: new Text("SHOW BOTTOM SHEET")
+                    )
+                )
+            );
+        }
+    }
+}

Samples/UIWidgetsGallery/demo/material/persistent_bottom_sheet_demo.cs.meta

+fileFormatVersion: 2
+guid: afb410d95c524a18b8761713f1335427
+timeCreated: 1555059936