Implement Banner.

Samples/UIWidgetsGallery/gallery/home.cs

                                 switchOutCurve: switchOutCurve,
                                 switchInCurve: switchInCurve,
                                 child: this._category == null
-                                    ? new _UIWidgetsLogo()
+                                    ? (Widget) new _UIWidgetsLogo()
                                     : new IconButton(
                                         icon: new BackButtonIcon(),
                                         tooltip: "Back",
                                 switchInCurve: switchInCurve,
                                 layoutBuilder: centerHome ? _centerHomeLayout : _topHomeLayout,
                                 child: this._category != null
-                                    ? new _DemosPage(_category)
+                                    ? (Widget) new _DemosPage(this._category)
                                     : new _CategoriesPage(
                                         categories: DemoUtils.kAllGalleryDemoCategories,
                                         onCategoryTap: (GalleryDemoCategory category) => {

Runtime/widgets/banner.cs

+using System;
+using Unity.UIWidgets.foundation;
+using Unity.UIWidgets.painting;
+using Unity.UIWidgets.ui;
+using Unity.UIWidgets.widgets;
+using UnityEngine;
+using Canvas = Unity.UIWidgets.ui.Canvas;
+using Color = Unity.UIWidgets.ui.Color;
+using Rect = Unity.UIWidgets.ui.Rect;
+using TextStyle = Unity.UIWidgets.painting.TextStyle;
+
+namespace Unity.UIWidgets.widgets {
+    class BannerConstants {
+        public const float _kOffset = 40.0f;
+        public const float _kHeight = 12.0f;
+        public const float _kBottomOffset = _kOffset + 0.707f * _kHeight;
+        public static readonly Rect _kRect = Rect.fromLTWH(-_kOffset, _kOffset - _kHeight, _kOffset * 2.0f, _kHeight);
+
+        public static readonly Color _kColor = new Color(0xA0B71C1C);
+        public static readonly TextStyle _kTextStyle = new TextStyle(
+          color: new Color(0xFFFFFFFF),
+          fontSize: _kHeight * 0.85f,
+          fontWeight: FontWeight.w700,
+          height: 1.0f
+        );
+    }
+
+    public enum BannerLocation {
+      topStart,
+
+      topEnd,
+
+      bottomStart,
+
+      bottomEnd,
+    }
+
+    public class BannerPainter : AbstractCustomPainter {
+      public BannerPainter(
+        string message,
+        BannerLocation? location,
+        Color color = null,
+        TextStyle textStyle = null
+      ) {D.assert(message != null);
+           D.assert(location != null);
+           D.assert(textStyle != null);
+           this.color = color ?? BannerConstants._kColor;
+           this.message = message;
+           this.location = location;
+           this.textStyle = textStyle ?? BannerConstants._kTextStyle;
+      }
+
+      public readonly string message;
+
+      public readonly BannerLocation? location;
+
+      public readonly Color color;
+
+      public readonly TextStyle textStyle;
+
+      readonly BoxShadow _shadow = new BoxShadow(
+        color: new Color(0x7F000000),
+        blurRadius: 6.0f
+      );
+
+      bool _prepared = false;
+      TextPainter _textPainter;
+      Paint _paintShadow;
+      Paint _paintBanner;
+
+      void _prepare() {
+          this._paintShadow = this._shadow.toPaint();
+          this._paintBanner = new Paint();
+          this._paintBanner.color = this.color;
+          this._textPainter = new TextPainter(
+              text: new TextSpan(style: this.textStyle, text: this.message),
+              textAlign: TextAlign.center
+            );
+          this._prepared = true;
+      }
+
+      public override void paint(Canvas canvas, Size size) {
+        if (!this._prepared) this._prepare();
+        canvas.translate(this._translationX(size.width), this._translationY(size.height));
+        canvas.rotate(this._rotation);
+        canvas.drawRect(BannerConstants._kRect, this._paintShadow);
+        canvas.drawRect(BannerConstants._kRect, this._paintBanner);
+        const float width = BannerConstants._kOffset * 2.0f;
+        this._textPainter.layout(minWidth: width, maxWidth: width);
+        this._textPainter.paint(canvas, BannerConstants._kRect.topLeft + new Offset(0.0f, (BannerConstants._kRect.height - this._textPainter.height) / 2.0f));
+      }
+
+      public override bool shouldRepaint(CustomPainter _oldDelegate) {
+          BannerPainter oldDelegate = _oldDelegate as BannerPainter;
+        return this.message != oldDelegate.message
+            || this.location != oldDelegate.location
+            || this.color != oldDelegate.color
+            || this.textStyle != oldDelegate.textStyle;
+      }
+
+      public override bool? hitTest(Offset position) => false;
+
+      float _translationX(float width) {
+            switch (this.location) {
+              case BannerLocation.bottomEnd:
+                return width - BannerConstants._kBottomOffset;
+              case BannerLocation.topEnd:
+                return width;
+              case BannerLocation.bottomStart:
+                return BannerConstants._kBottomOffset;
+              case BannerLocation.topStart:
+                return 0.0f;
+          default:
+              throw new Exception("Unknown location: " + this.location);
+        }
+      }
+
+      float _translationY(float height) {
+        D.assert(this.location != null);
+        switch (this.location) {
+          case BannerLocation.bottomStart:
+          case BannerLocation.bottomEnd:
+            return height - BannerConstants._kBottomOffset;
+          case BannerLocation.topStart:
+          case BannerLocation.topEnd:
+            return 0.0f;
+          default:
+              throw new Exception("Unknown location: " + this.location);
+        }
+      }
+
+      float _rotation {
+          get {
+              switch (this.location) {
+                  case BannerLocation.bottomStart:
+                  case BannerLocation.topEnd:
+                      return Mathf.PI / 4.0f;
+                  case BannerLocation.bottomEnd:
+                  case BannerLocation.topStart:
+                      return -Mathf.PI / 4.0f;
+                  default:
+                      throw new Exception("Unknown location: " + this.location);
+              }
+          }
+      }
+    }
+
+    public class Banner : StatelessWidget {
+      public Banner(
+        Key key = null,
+        Widget child = null,
+        string message = null,
+        BannerLocation? location = null,
+        Color color = null,
+        TextStyle textStyle = null
+      ) : base(key: key) {
+           D.assert(message != null);
+           D.assert(color != null);
+           D.assert(textStyle != null);
+           this.child = child;
+           this.message = message;
+           this.location = location;
+           this.color = color ?? BannerConstants._kColor;
+           this.textStyle = textStyle ?? BannerConstants._kTextStyle;
+      }
+
+      public readonly Widget child;
+
+      public readonly String message;
+
+      public readonly BannerLocation? location;
+
+      public readonly Color color;
+
+      public readonly TextStyle textStyle;
+
+      public override Widget build(BuildContext context) {
+        D.assert(WidgetsD.debugCheckHasDirectionality(context));
+        return new CustomPaint(
+          foregroundPainter: new BannerPainter(
+            message: this.message,
+            location: this.location,
+            color: this.color,
+            textStyle: this.textStyle
+          ),
+          child: this.child
+        );
+      }
+
+      public override void debugFillProperties(DiagnosticPropertiesBuilder properties) {
+        base.debugFillProperties(properties);
+        properties.add(new StringProperty("message", this.message, showName: false));
+        properties.add(new EnumProperty<BannerLocation?>("location", this.location));
+        properties.add(new DiagnosticsProperty<Color>("color", this.color, showName: false));
+        this.textStyle?.debugFillProperties(properties);
+      }
+    }
+
+    public class CheckedModeBanner : StatelessWidget {
+        public CheckedModeBanner(
+            Key key = null,
+            Widget child = null
+        ) : base(key: key) {
+            D.assert(child != null);
+            this.child = child;
+        }
+      
+
+      public readonly Widget child;
+
+      public override Widget build(BuildContext context) {
+        Widget result = this.child;
+        D.assert(() => {
+          result = new Banner(
+            child: result,
+            message: "DEBUG",
+            location: BannerLocation.topEnd
+          );
+          return true;
+        });
+        return result;
+      }
+
+      public override void debugFillProperties(DiagnosticPropertiesBuilder properties) {
+        base.debugFillProperties(properties);
+        String message = "disabled";
+        D.assert(() => {
+          message = "'DEBUG'";
+          return true;
+        });
+        properties.add(DiagnosticsNode.message(message));
+      }
+    }
+}

Runtime/widgets/banner.cs.meta

+fileFormatVersion: 2
+guid: e2e859ca92594b5d98bcce5ce505ca7b
+timeCreated: 1552882905