Make copy of dictionary.

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

@@ -3,6 +3,7 @@
 using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
+using System.Linq;
 
 namespace Roslyn.Utilities
 {
@@ -58,7 +59,9 @@ public static ObjectBinderState AllocateStateCopy()
 
                 // Otherwise, create copy from our current state and return that.
                 var state = new ObjectBinderState(
-                    s_version, s_typeToIndex, s_types.ToImmutableArray(), s_typeReaders.ToImmutableArray());
+                    s_version, 
+                    s_typeToIndex.ToDictionary(kvp => kvp.Key, kvp.kvp.Value),
+                    s_types.ToImmutableArray(), s_typeReaders.ToImmutableArray());
 
                 return state;
             }