Fix compile error.

src/Avalonia.Base/Collections/AvaloniaDictionary.cs

@@ -62,8 +62,6 @@ namespace Avalonia.Collections
 
         public object SyncRoot => ((IDictionary)_inner).SyncRoot;
 
-        public object this[object key] { get => ((IDictionary)_inner)[key]; set => ((IDictionary)_inner)[key] = value; }
-
         /// <summary>
         /// Gets or sets the named resource.
         /// </summary>
@@ -102,6 +100,8 @@ namespace Avalonia.Collections
             }
         }
 
+        object IDictionary.this[object key] { get => ((IDictionary)_inner)[key]; set => ((IDictionary)_inner)[key] = value; }
+
         /// <inheritdoc/>
         public void Add(TKey key, TValue value)
         {