Use a consistent comparison method for BKTrees and EditDistance.

src/Workspaces/Core/Portable/FindSymbols/SymbolTree/SymbolTreeInfo.cs

@@ -7,6 +7,7 @@
 using System.Threading;
 using System.Threading.Tasks;
 using Roslyn.Utilities;
+using static Roslyn.Utilities.PortableShim;
 
 namespace Microsoft.CodeAnalysis.FindSymbols
 {
@@ -236,7 +237,6 @@ internal static SymbolTreeInfo Create(VersionStamp version, IAssemblySymbol asse
             GenerateNodes(assembly.GlobalNamespace, list);
 
             var spellChecker = new SpellChecker(list.Select(n => n.Name));
-
             return new SymbolTreeInfo(version, SortNodes(list), spellChecker);
         }
 

src/Workspaces/Core/Portable/FindSymbols/SymbolTree/SymbolTreeInfo_Serialization.cs

@@ -13,7 +13,7 @@ namespace Microsoft.CodeAnalysis.FindSymbols
     internal partial class SymbolTreeInfo : IObjectWritable
     {
         private const string PrefixMetadataSymbolTreeInfo = "<MetadataSymbolTreeInfoPersistence>_";
-        private const string SerializationFormat = "8";
+        private const string SerializationFormat = "9";
 
         /// <summary>
         /// this is for a metadata reference in a solution

src/Workspaces/Core/Portable/Utilities/BKTree.Builder.cs

@@ -105,7 +105,7 @@ public Builder(IEnumerable<string> values)
 
                     foreach (var ch in value)
                     {
-                        _concatenatedLowerCaseWords[characterIndex] = char.ToLower(ch);
+                        _concatenatedLowerCaseWords[characterIndex] = CaseInsensitiveComparison.ToLower(ch);
                         characterIndex++;
                     }
                 }

src/Workspaces/Core/Portable/Utilities/BKTree.cs

@@ -4,6 +4,7 @@
 using System.Collections.Immutable;
 using System.Linq;
 using System.Text;
+using Microsoft.CodeAnalysis;
 using Roslyn.Utilities;
 using static System.Math;
 
@@ -82,7 +83,7 @@ public IList<string> Find(string value, int? threshold = null)
             {
                 for (var i = 0; i < value.Length; i++)
                 {
-                    lowerCaseCharacters[i] = char.ToLower(value[i]);
+                    lowerCaseCharacters[i] = CaseInsensitiveComparison.ToLower(value[i]);
                 }
 
                 threshold = threshold ?? WordSimilarityChecker.GetThreshold(value);

src/Workspaces/Core/Portable/Utilities/EditDistance.cs

@@ -4,6 +4,7 @@
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Text;
+using Microsoft.CodeAnalysis;
 using static System.Math;
 
 namespace Roslyn.Utilities
@@ -61,7 +62,7 @@ private static char[] ConvertToLowercaseArray(string text)
             var array = ArrayPool<char>.GetArray(text.Length);
             for (int i = 0; i < text.Length; i++)
             {
-                array[i] = char.ToLower(text[i]);
+                array[i] = CaseInsensitiveComparison.ToLower(text[i]);
             }
 
             return array;