addressed PR feedback

Author: vladima

src/compiler/tsc.ts

@@ -159,6 +159,8 @@ namespace ts {
         let timerHandleForRecompilation: number;                    // Handle for 0.25s wait timer to trigger recompilation
         let timerHandleForDirectoryChanges: number;                 // Handle for 0.25s wait timer to trigger directory change handler
 
+        // This map stores and reuses results of fileExists check that happen inside 'createProgram'
+        // This allows to save time in module resolution heavy scenarios when existence of the same file might be checked multiple times.
         let cachedExistingFiles: Map<boolean>;
         let hostFileExists: typeof compilerHost.fileExists;
 
@@ -279,7 +281,7 @@ namespace ts {
                 compilerHost.getSourceFile = getSourceFile;
 
                 hostFileExists = compilerHost.fileExists;
-                compilerHost.fileExists = fileExists;
+                compilerHost.fileExists = cachedFileExists;
             }
 
             // reset the cache of existing files
@@ -295,7 +297,7 @@ namespace ts {
             reportWatchDiagnostic(createCompilerDiagnostic(Diagnostics.Compilation_complete_Watching_for_file_changes));
         }
 
-        function fileExists(fileName: string): boolean {
+        function cachedFileExists(fileName: string): boolean {
             if (hasProperty(cachedExistingFiles, fileName)) {
                 return cachedExistingFiles[fileName];
             }