Temp store.

Runtime/material/button.cs

 using System;
 using Unity.UIWidgets.foundation;
+using Unity.UIWidgets.gestures;
 using Unity.UIWidgets.painting;
 using Unity.UIWidgets.rendering;
 using Unity.UIWidgets.ui;
                         onHighlightChanged: this._handleHighlightChanged,
                         splashColor: this.widget.splashColor,
                         highlightColor: this.widget.highlightColor,
-                        onTap: () => {
+                        onTap: this.widget.onPressed == null ? (GestureTapCallback) null : () => {
                             if (this.widget.onPressed != null) {
                                 this.widget.onPressed();
                             }

Runtime/material/outline_button.cs

                 child: child
             ) {
             D.assert(highlightElevation == null || highlightElevation >= 0.0f);
-            D.assert(onPressed != null);
             this.highlightedBorderColor = highlightedBorderColor;
             this.disabledBorderColor = disabledBorderColor;
             this.borderSide = borderSide;

Samples/UIWidgetsGallery/gallery/home.cs

                             frontTitle: new AnimatedSwitcher(
                                 duration: HomeUtils._kFrontLayerSwitchDuration,
                                 child: this._category == null
-                                    ? new Text("Flutter gallery")
+                                    ? new Text("UIWidgets gallery")
                                     : new Text(this._category.name)
                             ),
                             frontHeading: this.widget.testMode ? null : new Container(height: 24.0f),