fix menuitem command property.

src/Avalonia.Controls/Button.cs

@@ -32,6 +32,8 @@ namespace Avalonia.Controls
     /// </summary>
     public class Button : ContentControl
     {
+        private ICommand _command;
+
         /// <summary>
         /// Defines the <see cref="ClickMode"/> property.
         /// </summary>
@@ -69,8 +71,6 @@ namespace Avalonia.Controls
         public static readonly RoutedEvent<RoutedEventArgs> ClickEvent =
             RoutedEvent.Register<Button, RoutedEventArgs>(nameof(Click), RoutingStrategies.Bubble);
 
-        private ICommand _command;
-
         public static readonly StyledProperty<bool> IsPressedProperty =
             AvaloniaProperty.Register<Button, bool>(nameof(IsPressed));
 

src/Avalonia.Controls/MenuItem.cs

@@ -20,11 +20,13 @@ namespace Avalonia.Controls
     /// </summary>
     public class MenuItem : HeaderedSelectingItemsControl, IMenuItem, ISelectable
     {
+        private ICommand _command;
+
         /// <summary>
         /// Defines the <see cref="Command"/> property.
         /// </summary>
-        public static readonly StyledProperty<ICommand> CommandProperty =
-            AvaloniaProperty.Register<MenuItem, ICommand>(nameof(Command));
+        public static readonly DirectProperty<MenuItem, ICommand> CommandProperty =
+            Button.CommandProperty.AddOwner<MenuItem>(menuItem => menuItem.Command, (menuItem, command) => menuItem.Command = command, enableDataValidation: true);
 
         /// <summary>
         /// Defines the <see cref="HotKey"/> property.
@@ -159,8 +161,8 @@ namespace Avalonia.Controls
         /// </summary>
         public ICommand Command
         {
-            get { return GetValue(CommandProperty); }
-            set { SetValue(CommandProperty, value); }
+            get { return _command; }
+            set { SetAndRaise(CommandProperty, ref _command, value); }
         }
 
         /// <summary>