Make fields readonly.

And remove a `using` that crept in unexpectedly.

src/Avalonia.Base/PropertyStore/AvaloniaPropertyDictionaryPool.cs

@@ -6,7 +6,7 @@ namespace Avalonia.PropertyStore
     internal static class AvaloniaPropertyDictionaryPool<TValue>
     {
         private const int MaxPoolSize = 4;
-        private static Stack<AvaloniaPropertyDictionary<TValue>> _pool = new();
+        private static readonly Stack<AvaloniaPropertyDictionary<TValue>> _pool = new();
 
         public static AvaloniaPropertyDictionary<TValue> Get()
         {

src/Avalonia.Base/PropertyStore/BindingEntryBase.cs

@@ -11,8 +11,8 @@ namespace Avalonia.PropertyStore
         IObserver<BindingValue<TSource>>,
         IDisposable
     {
-        private static IDisposable s_creating = Disposable.Empty;
+        private static readonly IDisposable s_creating = Disposable.Empty;
-        private static IDisposable s_creatingQuiet = Disposable.Create(() => { });
+        private static readonly IDisposable s_creatingQuiet = Disposable.Create(() => { });
         private IDisposable? _subscription;
         private bool _hasValue;
         private TValue? _value;

src/Avalonia.Base/Utilities/AvaloniaPropertyDictionary.cs

@@ -1,7 +1,6 @@
 using System;
 using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
-using static Avalonia.Rendering.Composition.Animations.PropertySetSnapshot;
 
 namespace Avalonia.Utilities
 {