Fields can't be immutable.

src/Compilers/Core/Portable/Serialization/StreamObjectReader.cs

@@ -49,7 +49,7 @@ internal sealed partial class ObjectReader : IDisposable
         /// that means that <see cref="ObjectBinder"/> is both static and locked.  This gives us 
         /// local copy we can work with without needing to worry about anyone else mutating.
         /// </summary>
-        private readonly ObjectBinderState _binderState;
+        private ObjectBinderState _binderState;
 
         private int _recursionDepth;
 

src/Compilers/Core/Portable/Serialization/StreamObjectWriter.cs

@@ -42,7 +42,7 @@ internal sealed partial class ObjectWriter : IDisposable
         /// that means that <see cref="ObjectBinder"/> is both static and locked.  This gives us 
         /// local copy we can work with without needing to worry about anyone else mutating.
         /// </summary>
-        private readonly ObjectBinderState _binderState;
+        private ObjectBinderState _binderState;
 
         private int _recursionDepth;
         internal const int MaxRecursionDepth = 50;