Fix unsupported API usage in .NET Framework

Author: rjmholt

src/PowerShellEditorServices/Language/LanguageService.cs

@@ -310,9 +310,14 @@ public async Task<FindReferencesResult> FindReferencesOfSymbol(
 
             // We want to look for references first in referenced files, hence we use ordered dictionary
             // TODO: File system case-sensitivity is based on filesystem not OS, but OS is a much cheaper heuristic
+#if CoreCLR
+            // The RuntimeInformation.IsOSPlatform is not supported in .NET Framework
             var fileMap = RuntimeInformation.IsOSPlatform(OSPlatform.Linux)
                 ? new OrderedDictionary()
                 : new OrderedDictionary(StringComparer.OrdinalIgnoreCase);
+#else
+            var fileMap = new OrderedDictionary(StringComparer.OrdinalIgnoreCase);
+#endif
             foreach (ScriptFile file in referencedFiles)
             {
                 fileMap.Add(file.FilePath, file);