Fix build break in closed project sharing source file

src/Compilers/Core/Portable/InternalUtilities/SpecializedCollections.Empty.Set.cs

@@ -9,7 +9,10 @@ internal partial class SpecializedCollections
     {
         private partial class Empty
         {
-            internal class Set<T> : Collection<T>, ISet<T>, IReadOnlySet<T>
+            internal class Set<T> : Collection<T>, ISet<T>
+#if COMPILERCORE
+                , IReadOnlySet<T>
+#endif
             {
                 public static readonly new Set<T> Instance = new Set<T>();
 

src/Compilers/Core/Portable/InternalUtilities/SpecializedCollections.ReadOnly.Set.cs

@@ -9,7 +9,10 @@ internal partial class SpecializedCollections
     {
         private partial class ReadOnly
         {
-            internal class Set<TUnderlying, T> : Collection<TUnderlying, T>, ISet<T>, IReadOnlySet<T>
+            internal class Set<TUnderlying, T> : Collection<TUnderlying, T>, ISet<T>
+#if COMPILERCORE
+                , IReadOnlySet<T>
+#endif
                 where TUnderlying : ISet<T>
             {
                 public Set(TUnderlying underlying)

src/Compilers/Core/Portable/InternalUtilities/SpecializedCollections.cs

@@ -44,10 +44,12 @@ public static ISet<T> EmptySet<T>()
             return Empty.Set<T>.Instance;
         }
 
+#if COMPILERCORE
         public static IReadOnlySet<T> EmptyReadOnlySet<T>()
         {
             return Empty.Set<T>.Instance;
         }
+#endif
 
         public static IDictionary<TKey, TValue> EmptyDictionary<TKey, TValue>()
         {
@@ -98,11 +100,13 @@ public static ISet<T> ReadOnlySet<T>(ISet<T> set)
                 : new ReadOnly.Set<ISet<T>, T>(set);
         }
 
+#if COMPILERCORE
         public static IReadOnlySet<T> StronglyTypedReadOnlySet<T>(ISet<T> set)
         {
             return set == null || set.Count == 0
                 ? EmptyReadOnlySet<T>()
                 : new ReadOnly.Set<ISet<T>, T>(set);
         }
+#endif
     }
 }