performance overlay support on WidgetApp/MaterialApp

Runtime/widgets/app.cs

         public readonly Dictionary<string, WidgetBuilder> routes;
         public readonly TextStyle textStyle;
         public readonly Window window;
+        public readonly bool showPerformanceoverlay;
 
         public WidgetsApp(
             Key key = null,
             Dictionary<string, WidgetBuilder> routes = null,
             TransitionBuilder builder = null,
             TextStyle textStyle = null,
-            Widget home = null
+            Widget home = null,
+            bool showPerformanceOverlay = false
         ) : base(key) {
             routes = routes ?? new Dictionary<string, WidgetBuilder>();
             this.window = Window.instance;
             this.initialRoute = initialRoute;
             this.builder = builder;
             this.textStyle = textStyle;
+            this.showPerformanceoverlay = showPerformanceOverlay;
 
             D.assert(
                 home == null ||
                     style: this.widget.textStyle,
                     child: result
                 );
+            }
+
+            PerformanceOverlay performanceOverlay = null;
+            if (this.widget.showPerformanceoverlay) {
+                performanceOverlay = PerformanceOverlay.allEnabled();
+            }
+
+            if (performanceOverlay != null) {
+                result = new Stack(
+                    children: new List<Widget> {
+                        result,
+                        new Positioned(top: 0.0f,
+                            left: 0.0f,
+                            right: 0.0f,
+                            child: performanceOverlay)
+                    });
             }
 
             D.assert(() => {