Update.

Samples/UIWidgetsGallery/gallery/demos.cs

                     routeName: ButtonsDemo.routeName,
                     buildRoute: (BuildContext context) => new ButtonsDemo()
                 ),
-//                new GalleryDemo(
-//                    title: "Buttons: Floating Action Button",
-//                    subtitle: "FAB with transitions",
-//                    icon: GalleryIcons.buttons,
-//                    category: GalleryDemoCategory._kMaterialComponents,
-//                    routeName: TabsFabDemo.routeName,
-//                    documentationUrl: "https://docs.flutter.io/flutter/material/FloatingActionButton-class.html",
-//                    buildRoute: (BuildContext context) => TabsFabDemo()
-//                ),
+                new GalleryDemo(
+                    title: "Buttons: Floating Action Button",
+                    subtitle: "FAB with transitions",
+                    icon: GalleryIcons.buttons,
+                    category: _kMaterialComponents,
+                    routeName: TabsFabDemo.routeName,
+                    documentationUrl: "https://docs.flutter.io/flutter/material/FloatingActionButton-class.html",
+                    buildRoute: (BuildContext context) => new TabsFabDemo()
+                ),
                 new GalleryDemo(
                     title: "Cards",
                     subtitle: "Baseline cards with rounded corners",

Samples/UIWidgetsGallery/demo/material/tabs_fab_demo.cs

+using System.Collections.Generic;
+using System.Linq;
+using Unity.UIWidgets.foundation;
+using Unity.UIWidgets.material;
+using Unity.UIWidgets.painting;
+using Unity.UIWidgets.ui;
+using Unity.UIWidgets.widgets;
+
+namespace UIWidgetsGallery.gallery {
+    class TabsFabDemoUtils {
+        public const string _explanatoryText =
+            "When the Scaffold's floating action button changes, the new button fades and " +
+            "turns into view. In this demo, changing tabs can cause the app to be rebuilt " +
+            "with a FloatingActionButton that the Scaffold distinguishes from the others " +
+            "by its key.";
+
+        public static readonly List<_Page> _allPages = new List<_Page> {
+            new _Page(label: "Blue", colors: Colors.indigo, icon: Icons.add),
+            new _Page(label: "Eco", colors: Colors.green, icon: Icons.create),
+            new _Page(label: "No"),
+            new _Page(label: "Teal", colors: Colors.teal, icon: Icons.add),
+            new _Page(label: "Red", colors: Colors.red, icon: Icons.create)
+        };
+    }
+
+    class _Page {
+        public _Page(
+            string label,
+            MaterialColor colors = null,
+            IconData icon = null
+        ) {
+            this.label = label;
+            this.colors = colors;
+            this.icon = icon;
+        }
+
+        public readonly string label;
+        public readonly MaterialColor colors;
+        public readonly IconData icon;
+
+        public Color labelColor {
+            get { return this.colors != null ? this.colors.shade300 : Colors.grey.shade300; }
+        }
+
+        public bool fabDefined {
+            get { return this.colors != null && this.icon != null; }
+        }
+
+        public Color fabColor {
+            get { return this.colors.shade400; }
+        }
+
+        public Icon fabIcon {
+            get { return new Icon(this.icon); }
+        }
+
+        public Key fabKey {
+            get { return new ValueKey<Color>(this.fabColor); }
+        }
+    }
+
+
+    public class TabsFabDemo : StatefulWidget {
+        public const string routeName = "/material/tabs-fab";
+
+        public override State createState() {
+            return new _TabsFabDemoState();
+        }
+    }
+
+    class _TabsFabDemoState : SingleTickerProviderStateMixin<TabsFabDemo> {
+        GlobalKey<ScaffoldState> _scaffoldKey = GlobalKey<ScaffoldState>.key();
+
+        TabController _controller;
+        _Page _selectedPage;
+        bool _extendedButtons = false;
+
+        public override void initState() {
+            base.initState();
+            this._controller = new TabController(vsync: this, length: TabsFabDemoUtils._allPages.Count);
+            this._controller.addListener(this._handleTabSelection);
+            this._selectedPage = TabsFabDemoUtils._allPages[0];
+        }
+
+        public override void dispose() {
+            this._controller.dispose();
+            base.dispose();
+        }
+
+        void _handleTabSelection() {
+            this.setState(() => { this._selectedPage = TabsFabDemoUtils._allPages[this._controller.index]; });
+        }
+
+        void _showExplanatoryText() {
+            this._scaffoldKey.currentState.showBottomSheet((BuildContext context) => {
+                return new Container(
+                    decoration: new BoxDecoration(
+                        border: new Border(top: new BorderSide(color: Theme.of(this.context).dividerColor))
+                    ),
+                    child: new Padding(
+                        padding: EdgeInsets.all(32.0f),
+                        child: new Text(TabsFabDemoUtils._explanatoryText,
+                            style: Theme.of(this.context).textTheme.subhead)
+                    )
+                );
+            });
+        }
+
+        Widget buildTabView(_Page page) {
+            return new Builder(
+                builder: (BuildContext context) => {
+                    return new Container(
+                        key: new ValueKey<string>(page.label),
+                        padding: EdgeInsets.fromLTRB(48.0f, 48.0f, 48.0f, 96.0f),
+                        child: new Card(
+                            child: new Center(
+                                child: new Text(page.label,
+                                    style: new TextStyle(
+                                        color: page.labelColor,
+                                        fontSize: 32.0f
+                                    ),
+                                    textAlign: TextAlign.center
+                                )
+                            )
+                        )
+                    );
+                }
+            );
+        }
+
+        Widget buildFloatingActionButton(_Page page) {
+            if (!page.fabDefined) {
+                return null;
+            }
+
+            if (this._extendedButtons) {
+                return FloatingActionButton.extended(
+                    key: new ValueKey<Key>(page.fabKey),
+                    tooltip: "Show explanation",
+                    backgroundColor: page.fabColor,
+                    icon: page.fabIcon,
+                    label: new Text(page.label.ToUpper()),
+                    onPressed: this._showExplanatoryText
+                );
+            }
+
+            return new FloatingActionButton(
+                key: page.fabKey,
+                tooltip: "Show explanation",
+                backgroundColor: page.fabColor,
+                child: page.fabIcon,
+                onPressed: this._showExplanatoryText
+            );
+        }
+
+        public override Widget build(BuildContext context) {
+            return new Scaffold(
+                key: this._scaffoldKey,
+                appBar: new AppBar(
+                    title: new Text("FAB per tab"),
+                    bottom: new TabBar(
+                        controller: this._controller,
+                        tabs: TabsFabDemoUtils._allPages
+                            .Select<_Page, Widget>((_Page page) => new Tab(text: page.label.ToUpper())).ToList()
+                    ),
+                    actions: new List<Widget> {
+                        new MaterialDemoDocumentationButton(TabsFabDemo.routeName),
+                        new IconButton(
+                            icon: new Icon(Icons.sentiment_very_satisfied),
+                            onPressed: () => {
+                                this.setState(() => { this._extendedButtons = !this._extendedButtons; });
+                            }
+                        )
+                    }
+                ),
+                floatingActionButton: this.buildFloatingActionButton(this._selectedPage),
+                body: new TabBarView(
+                    controller: this._controller,
+                    children: TabsFabDemoUtils._allPages.Select<_Page, Widget>(this.buildTabView).ToList()
+                )
+            );
+        }
+    }
+}

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

+fileFormatVersion: 2
+guid: 7c2dcb794cc84a458f028f3751cf08a7
+timeCreated: 1555060923