Fix build

Author: davidwengier

src/Razor/test/Microsoft.AspNetCore.Razor.LanguageServer.Test/IFileSystemExtensionsTest.cs

@@ -103,5 +103,8 @@ public bool FileExists(string filePath)
 
         public string ReadFile(string filePath)
             => throw new NotImplementedException();
+
+        public Stream ReadStream(string filePath)
+            => throw new NotImplementedException();
     }
 }