Add copyright header, make property readonly

src/EditorFeatures/Test/TextEditorFactoryExtensions.cs

-using Microsoft.VisualStudio.Text;
+// Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
+
+using Microsoft.VisualStudio.Text;
 using Microsoft.VisualStudio.Text.Editor;
 using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Text;
-using System.Threading.Tasks;
 
 namespace Microsoft.CodeAnalysis.Editor.UnitTests
 {
@@ -28,7 +26,7 @@ public DisposableTextView(IWpfTextView textView)
             this.TextView = textView;
         }
 
-        public IWpfTextView TextView { get; private set; }
+        public IWpfTextView TextView { get; }
 
         public void Dispose()
         {