Implement FlatButton and demo

Runtime/material/button_theme.cs

         }
 
 
-        Color getFillColor(MaterialButton button) {
+        public Color getFillColor(MaterialButton button) {
             Color fillColor = button.enabled ? button.color : button.disabledColor;
             if (fillColor != null) {
                 return fillColor;

Samples/MaterialSample/DividerButtonCanvas.cs

 using TextStyle = Unity.UIWidgets.painting.TextStyle;
 
 namespace UIWidgetsSample {
-    public class DividerDemo : WidgetCanvas {
+    public class DividerButtonCanvas : WidgetCanvas {
         protected override Widget getWidget() {
             return new DemoApp();
         }

Runtime/material/flat_button.cs

+using System;
+using System.Collections.Generic;
+using Unity.UIWidgets.rendering;
+using Unity.UIWidgets.foundation;
+using Unity.UIWidgets.painting;
+using Unity.UIWidgets.service;
+using Unity.UIWidgets.ui;
+using Unity.UIWidgets.widgets;
+
+namespace Unity.UIWidgets.material {
+    public class FlatButton : MaterialButton {
+        public FlatButton(
+            Key key = null,
+            VoidCallback onPressed = null,
+            ValueChanged<bool> onHighlightChanged = null,
+            ButtonTextTheme? textTheme = null,
+            Color textColor = null,
+            Color disabledTextColor = null,
+            Color color = null,
+            Color disabledColor = null,
+            Color highlightColor = null,
+            Color splashColor = null,
+            Brightness? colorBrightness = null,
+            EdgeInsets padding = null,
+            ShapeBorder shape = null,
+            Clip clipBehavior = Clip.none,
+            MaterialTapTargetSize? materialTapTargetSize = null,
+            Widget child = null) : base(
+            key: key,
+            onPressed: onPressed,
+            onHighlightChanged: onHighlightChanged,
+            textTheme: textTheme,
+            textColor: textColor,
+            disabledTextColor: disabledTextColor,
+            color: color,
+            disabledColor: disabledColor,
+            highlightColor: highlightColor,
+            splashColor: splashColor,
+            colorBrightness: colorBrightness,
+            elevation: null,
+            highlightElevation: null,
+            disabledElevation: null,
+            padding: padding,
+            shape: shape,
+            clipBehavior: clipBehavior,
+            materialTapTargetSize: materialTapTargetSize,
+            animationDuration: null,
+            minWidth: null,
+            height: null,
+            child: child) {}
+
+        public static FlatButton icon(
+            Key key = null,
+            VoidCallback onPressed = null,
+            ValueChanged<bool> onHighlightChanged = null,
+            ButtonTextTheme? textTheme = null,
+            Color textColor = null,
+            Color disabledTextColor = null,
+            Color color = null,
+            Color disabledColor = null,
+            Color highlightColor = null,
+            Color splashColor = null,
+            Brightness? colorBrightness = null,
+            EdgeInsets padding = null,
+            ShapeBorder shape = null,
+            Clip clipBehavior = Clip.none,
+            MaterialTapTargetSize? materialTapTargetSize = null,
+            Widget icon = null,
+            Widget label = null) {
+            return new _FlatButtonWithIcon(
+                key: key,
+                onPressed: onPressed,
+                onHighlightChanged: onHighlightChanged,
+                textTheme: textTheme,
+                textColor: textColor,
+                disabledTextColor: disabledTextColor,
+                color: color,
+                disabledColor: disabledColor,
+                highlightColor: highlightColor,
+                splashColor: splashColor,
+                colorBrightness: colorBrightness,
+                padding: padding,
+                shape: shape,
+                clipBehavior: clipBehavior,
+                materialTapTargetSize: materialTapTargetSize,
+                icon: icon,
+                label: label
+            );
+        }
+
+        public override Widget build(BuildContext context) {
+            ThemeData theme = Theme.of(context);
+            ButtonThemeData buttonTheme = ButtonTheme.of(context);
+
+            return new RawMaterialButton(
+                onPressed: onPressed,
+                onHighlightChanged: onHighlightChanged,
+                clipBehavior: clipBehavior ?? Clip.none,
+                fillColor: buttonTheme.getFillColor(this),
+                textStyle: theme.textTheme.button.copyWith(color: buttonTheme.getTextColor(this)),
+                highlightColor: buttonTheme.getHighlightColor(this),
+                splashColor: buttonTheme.getSplashColor(this),
+                elevation: buttonTheme.getElevation(this),
+                highlightElevation: buttonTheme.getHighlightElevation(this),
+                disabledElevation: buttonTheme.getDisabledElevation(this),
+                padding: buttonTheme.getPadding(this),
+                constraints: buttonTheme.getConstraints(this),
+                shape: buttonTheme.getShape(this),
+                animationDuration: buttonTheme.getAnimationDuration(this),
+                materialTapTargetSize: buttonTheme.getMaterialTapTargetSize(this),
+                child: child
+            );
+        }
+
+        public override void debugFillProperties(DiagnosticPropertiesBuilder properties) {
+            base.debugFillProperties(properties);
+            properties.add(new ObjectFlagProperty<VoidCallback>("onPressed", onPressed, ifNull: "disabled"));
+            properties.add(new DiagnosticsProperty<ButtonTextTheme?>("textTheme", textTheme, defaultValue: null));
+            properties.add(new DiagnosticsProperty<Color>("textColor", textColor, defaultValue: null));
+            properties.add(new DiagnosticsProperty<Color>("disabledTextColor", disabledTextColor, defaultValue: null));
+            properties.add(new DiagnosticsProperty<Color>("color", color, defaultValue: null));
+            properties.add(new DiagnosticsProperty<Color>("disabledColor", disabledColor, defaultValue: null));
+            properties.add(new DiagnosticsProperty<Color>("highlightColor", highlightColor, defaultValue: null));
+            properties.add(new DiagnosticsProperty<Color>("splashColor", splashColor, defaultValue: null));
+            properties.add(new DiagnosticsProperty<Brightness?>("colorBrightness", colorBrightness, defaultValue: null));
+            properties.add(new DiagnosticsProperty<EdgeInsets>("padding", padding, defaultValue: null));
+            properties.add(new DiagnosticsProperty<ShapeBorder>("shape", shape, defaultValue: null));
+            properties.add(new DiagnosticsProperty<MaterialTapTargetSize?>("materialTapTargetSize", materialTapTargetSize, defaultValue: null));
+        }
+    }
+
+    class _FlatButtonWithIcon : FlatButton {
+        public _FlatButtonWithIcon(
+            Key key = null,
+            VoidCallback onPressed = null,
+            ValueChanged<bool> onHighlightChanged = null,
+            ButtonTextTheme? textTheme = null,
+            Color textColor = null,
+            Color disabledTextColor = null,
+            Color color = null,
+            Color disabledColor = null,
+            Color highlightColor = null,
+            Color splashColor = null,
+            Brightness? colorBrightness = null,
+            EdgeInsets padding = null,
+            ShapeBorder shape = null,
+            Clip clipBehavior = Clip.none,
+            MaterialTapTargetSize? materialTapTargetSize = null,
+            Widget icon = null,
+            Widget label = null) : base(
+            key: key,
+            onPressed: onPressed,
+            onHighlightChanged: onHighlightChanged,
+            textTheme: textTheme,
+            textColor: textColor,
+            disabledTextColor: disabledTextColor,
+            color: color,
+            disabledColor: disabledColor,
+            highlightColor: highlightColor,
+            splashColor: splashColor,
+            colorBrightness: colorBrightness,
+            padding: padding,
+            shape: shape,
+            clipBehavior: clipBehavior,
+            materialTapTargetSize: materialTapTargetSize,
+            child: new Row(
+                mainAxisSize: MainAxisSize.min,
+                children: new List<Widget> {
+                    icon,
+                    new SizedBox(width: 8.0),
+                    label
+                }
+            )) {
+            D.assert(icon != null);
+            D.assert(label != null);
+        }
+    }
+}