Fix Test.

tests/Avalonia.Base.UnitTests/Media/PenTests.cs

@@ -53,7 +53,10 @@ namespace Avalonia.Base.UnitTests.Media
             var raised = false;
 
             target.Invalidated += (s, e) => raised = true;
-            dashes.Dashes.Add(0.3);
+            dashes.Dashes = new AvaloniaList<double>
+            {
+                0.3
+            };
 
             Assert.True(raised);
         }