Fixed tests

src/Avalonia.Base/Media/ConicGradientBrush.cs

@@ -47,7 +47,7 @@ namespace Avalonia.Media
         }
 
         /// <inheritdoc/>
-        public IImmutableBrush ToImmutable()
+        public override IImmutableBrush ToImmutable()
         {
             return new ImmutableConicGradientBrush(this);
         }

src/Avalonia.Base/Media/GradientBrush.cs

@@ -12,7 +12,7 @@ namespace Avalonia.Media
     /// <summary>
     /// Base class for brushes that draw with a gradient.
     /// </summary>
-    public abstract class GradientBrush : Brush, IGradientBrush
+    public abstract class GradientBrush : Brush, IGradientBrush, IMutableBrush
     {
         /// <summary>
         /// Defines the <see cref="SpreadMethod"/> property.
@@ -92,5 +92,7 @@ namespace Avalonia.Media
         {
             RaiseInvalidated(EventArgs.Empty);
         }
+
+        public abstract IImmutableBrush ToImmutable();
     }
 }

src/Avalonia.Base/Media/LinearGradientBrush.cs

@@ -47,7 +47,7 @@ namespace Avalonia.Media
         }
 
         /// <inheritdoc/>
-        public IImmutableBrush ToImmutable()
+        public override IImmutableBrush ToImmutable()
         {
             return new ImmutableLinearGradientBrush(this);
         }

src/Avalonia.Base/Media/RadialGradientBrush.cs

@@ -67,7 +67,7 @@ namespace Avalonia.Media
         }
 
         /// <inheritdoc/>
-        public IImmutableBrush ToImmutable()
+        public override IImmutableBrush ToImmutable()
         {
             return new ImmutableRadialGradientBrush(this);
         }