cache results of fileExists check in default compiler host

src/compiler/program.ts

@@ -209,6 +209,7 @@ namespace ts {
     export function createCompilerHost(options: CompilerOptions, setParentNodes?: boolean): CompilerHost {
         let currentDirectory: string;
         let existingDirectories: Map<boolean> = {};
+        let existingFiles: Map<boolean> = {};
 
         function getCanonicalFileName(fileName: string): string {
             // if underlying system can distinguish between two files whose names differs only in cases then file name already in canonical form.
@@ -249,6 +250,14 @@ namespace ts {
             return false;
         }
 
+        function fileExists(fileName: string): boolean {
+            if (hasProperty(existingFiles, fileName)) {
+                return existingFiles[fileName];
+            }
+
+            return existingFiles[fileName] = sys.fileExists(fileName);
+        }
+
         function ensureDirectoriesExist(directoryPath: string) {
             if (directoryPath.length > getRootLength(directoryPath) && !directoryExists(directoryPath)) {
                 let parentDirectory = getDirectoryPath(directoryPath);
@@ -281,7 +290,7 @@ namespace ts {
             useCaseSensitiveFileNames: () => sys.useCaseSensitiveFileNames,
             getCanonicalFileName,
             getNewLine: () => newLine,
-            fileExists: fileName => sys.fileExists(fileName),
+            fileExists,
             readFile: fileName => sys.readFile(fileName)
         };
     }