Add project files.

Compat.Private.Serialization/Compat/Runtime/Serialization/IDataNode.cs

@@ -0,0 +1,21 @@
+using System;
+
+namespace Compat.Runtime.Serialization
+{
+    internal interface IDataNode
+    {
+        Type DataType { get; }
+        object Value { get; set; }  // boxes for primitives
+        string DataContractName { get; set; }
+        string DataContractNamespace { get; set; }
+        string ClrTypeName { get; set; }
+        string ClrAssemblyName { get; set; }
+        string Id { get; set; }
+        bool PreservesReferences { get; }
+
+        // NOTE: consider moving below APIs to DataNode<T> if IDataNode API is made public
+        void GetData(ElementData element);
+        bool IsFinalValue { get; set; }
+        void Clear();
+    }
+}