Add Widget placeholder

Runtime/widgets/placeholder.cs

+using System.Collections.Generic;
+using Unity.UIWidgets.foundation;
+using Unity.UIWidgets.ui;
+
+namespace Unity.UIWidgets.widgets
+{
+    internal class _Placeholderpainter : AbstractCustomPainter
+    {
+        public _Placeholderpainter(
+            Color color,
+            float strokeWidth = 0f
+        )
+        {
+            this.color = color;
+            this.strokeWidth = strokeWidth;
+        }
+
+
+        public readonly Color color;
+        public readonly float strokeWidth;
+
+        public override void paint(Canvas canvas, Size size)
+        {
+            Paint paint = new Paint();
+            paint.color = color;
+            paint.style = PaintingStyle.stroke;
+            paint.strokeWidth = strokeWidth;
+
+            Rect rect = Offset.zero & size;
+            Path path = new Path();
+            path.addRect(rect);
+            path.addPolygon(new List<Offset> {rect.topRight, rect.bottomLeft}, false);
+            path.addPolygon(new List<Offset> {rect.topLeft, rect.bottomRight}, false);
+
+            canvas.drawPath(path, paint);
+            return;
+        }
+
+        public override bool shouldRepaint(CustomPainter oldPainter)
+        {
+            return ((_Placeholderpainter) oldPainter).color != color ||
+                   ((_Placeholderpainter) oldPainter).strokeWidth != strokeWidth;
+        }
+
+        public override bool? hitTest(Offset position)
+        {
+            return false;
+        }
+    }
+
+    public class Placeholder : StatelessWidget
+    {
+        public Placeholder(
+            Key key = null,
+            Color color = null,
+            float strokeWidth = 2.0f,
+            float fallbackWidth = 400.0f,
+            float fallbackHeight = 400.0f
+        ) : base(key)
+        {
+            this.color = color ?? new Color(0xFF455A64);
+            this.strokeWidth = strokeWidth;
+            this.fallbackWidth = fallbackWidth;
+            this.fallbackHeight = fallbackHeight;
+        }
+
+        public readonly Color color;
+        public readonly float strokeWidth;
+        public readonly float fallbackWidth;
+        public readonly float fallbackHeight;
+
+        public override Widget build(BuildContext context)
+        {
+            return new LimitedBox(
+                maxWidth: fallbackWidth,
+                maxHeight: fallbackHeight,
+                child: new CustomPaint(
+                    size: Size.infinite,
+                    foregroundPainter: new _Placeholderpainter(
+                        color: color,
+                        strokeWidth: strokeWidth
+                    )
+                )
+            );
+        }
+    }
+}

Runtime/widgets/placeholder.cs.meta

+fileFormatVersion: 2
+guid: 5219ee543b8f9432cbc6c2eeeb7b351c
+MonoImporter:
+  externalObjects: {}
+  serializedVersion: 2
+  defaultReferences: []
+  executionOrder: 0
+  icon: {instanceID: 0}
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: