Exclude CompactPrefixTree from code coverage

src/Markdig/Helpers/CompactPrefixTree.cs

@@ -4,6 +4,7 @@ using System;
 using System.Collections;
 using System.Collections.Generic;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Runtime.CompilerServices;
 
 /*
@@ -25,6 +26,7 @@ namespace Markdig.Helpers
     /// <para>Something between a Trie and a full Radix tree, but stored linearly in memory</para>
     /// </summary>
     /// <typeparam name="TValue">The value associated with the key</typeparam>
+    [ExcludeFromCodeCoverage]
     internal sealed class CompactPrefixTree<TValue>
 //#if !LEGACY
 //        : IReadOnlyDictionary<string, TValue>, IReadOnlyList<KeyValuePair<string, TValue>>

src/Markdig/Helpers/ExcludeFromCodeCoverageAttribute.cs

@@ -0,0 +1,12 @@
+#if NET35 || UAP
+using System;
+
+namespace System.Diagnostics.CodeAnalysis
+{
+    [AttributeUsage(AttributeTargets.All, AllowMultiple = false, Inherited = false)]
+    public class ExcludeFromCodeCoverageAttribute : Attribute
+    {
+
+    }
+}
+#endif

src/Markdig/Helpers/ThrowHelper.cs

@@ -1,11 +1,13 @@
 using System;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 
 namespace Markdig.Helpers
 {
     /// <summary>
     /// Inspired by CoreLib, taken from https://github.com/MihaZupan/SharpCollections, cc @MihaZupan
     /// </summary>
+    [ExcludeFromCodeCoverage]
     internal static class ThrowHelper
     {
         public static void ThrowArgumentNullException(ExceptionArgument argument)