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Change from internal to private for unchecked methods.

Co-authored-by: Stephen Toub <[email protected]>

src/libraries/System.Collections.Immutable/src/System/Collections/Immutable/ImmutableList_1.Node.cs

@@ -199,7 +199,7 @@ internal ref readonly T ItemRef(int index)
                 return ref ItemRefUnchecked(index);
             }
 
-            internal ref readonly T ItemRefUnchecked(int index)
+            private ref readonly T ItemRefUnchecked(int index)
             {
                 Debug.Assert(_left != null && _right != null);
                 if (index < _left._count)

src/libraries/System.Collections.Immutable/src/System/Collections/Immutable/ImmutableSortedSet_1.Node.cs

@@ -265,7 +265,7 @@ internal ref readonly T ItemRef(int index)
                 return ref ItemRefUnchecked(index);
             }
 
-            internal ref readonly T ItemRefUnchecked(int index)
+            private ref readonly T ItemRefUnchecked(int index)
             {
                 Debug.Assert(_left != null && _right != null);