Replace the bespoke OrderedDictionary with a backport of System.Collections.Generic.OrderedDictionary

src/libraries/Common/src/System/Collections/Generic/EnumerableHelpers.cs

@@ -1,6 +1,8 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Diagnostics;
+
 namespace System.Collections.Generic
 {
     /// <summary>
@@ -25,6 +27,11 @@ internal static IEnumerator<T> GetEmptyEnumerator<T>() =>
         /// </returns>
         internal static T[] ToArray<T>(IEnumerable<T> source, out int length)
         {
+            // Copied from Array.MaxLength in System.Private.CoreLib/src/libraries/System.Private.CoreLib/src/System/Array.cs
+            const int ArrayMaxLength = 0X7FFFFFC7;
+#if NET6_0_OR_GREATER
+            Debug.Assert(Array.MaxLength == ArrayMaxLength);
+#endif
             if (source is ICollection<T> ic)
             {
                 int count = ic.Count;
@@ -64,9 +71,9 @@ internal static T[] ToArray<T>(IEnumerable<T> source, out int length)
                                 // constrain the length to be Array.MaxLength (this overflow check works because of the
                                 // cast to uint).
                                 int newLength = count << 1;
-                                if ((uint)newLength > Array.MaxLength)
+                                if ((uint)newLength > ArrayMaxLength)
                                 {
-                                    newLength = Array.MaxLength <= count ? count + 1 : Array.MaxLength;
+                                    newLength = ArrayMaxLength <= count ? count + 1 : ArrayMaxLength;
                                 }
 
                                 Array.Resize(ref arr, newLength);

src/libraries/System.Collections/src/System.Collections.csproj

@@ -6,6 +6,7 @@
     <IsPartialFacadeAssembly>true</IsPartialFacadeAssembly>
     <ContractTypesPartiallyMoved>true</ContractTypesPartiallyMoved>
     <UseCompilerGeneratedDocXmlFile>false</UseCompilerGeneratedDocXmlFile>
+    <DefineConstants>SYSTEM_COLLECTIONS;$(DefineConstants)</DefineConstants>
   </PropertyGroup>
 
   <ItemGroup>