Use toLowercase as utility function

src/compiler/commandLineParser.ts

@@ -2020,7 +2020,7 @@ namespace ts {
     export function getFileNamesFromConfigSpecs(spec: ConfigFileSpecs, basePath: string, options: CompilerOptions, host: ParseConfigHost, extraFileExtensions: ReadonlyArray<JsFileExtensionInfo> = []): ExpandResult {
         basePath = normalizePath(basePath);
 
-        const keyMapper = host.useCaseSensitiveFileNames ? caseSensitiveKeyMapper : caseInsensitiveKeyMapper;
+        const keyMapper = host.useCaseSensitiveFileNames ? identity : toLowerCase;
 
         // Literal file names (provided via the "files" array in tsconfig.json) are stored in a
         // file map with a possibly case insensitive key. We use this map later when when including
@@ -2224,24 +2224,6 @@ namespace ts {
         }
     }
 
-    /**
-     * Gets a case sensitive key.
-     *
-     * @param key The original key.
-     */
-    function caseSensitiveKeyMapper(key: string) {
-        return key;
-    }
-
-    /**
-     * Gets a case insensitive key.
-     *
-     * @param key The original key.
-     */
-    function caseInsensitiveKeyMapper(key: string) {
-        return key.toLowerCase();
-    }
-
     /**
      * Produces a cleaned version of compiler options with personally identifiying info (aka, paths) removed.
      * Also converts enum values back to strings.

src/compiler/core.ts

@@ -1455,6 +1455,9 @@ namespace ts {
     /** Returns its argument. */
     export function identity<T>(x: T) { return x; }
 
+    /** Returns the lower case string */
+    export function toLowerCase(x: string) { return x.toLowerCase(); }
+
     /** Throws an error because a function is not implemented. */
     export function notImplemented(): never {
         throw new Error("Not implemented");
@@ -2925,9 +2928,7 @@ namespace ts {
 
     export type GetCanonicalFileName = (fileName: string) => string;
     export function createGetCanonicalFileName(useCaseSensitiveFileNames: boolean): GetCanonicalFileName {
-        return useCaseSensitiveFileNames
-            ? ((fileName) => fileName)
-            : ((fileName) => fileName.toLowerCase());
+        return useCaseSensitiveFileNames ? identity : toLowerCase;
     }
 
     /**