type WriteFileCallback: make param sourceFiles required. (microsoft#18731)

Fixes microsoft#15007

Author: NaridaL

src/compiler/types.ts

@@ -2442,7 +2442,7 @@ namespace ts {
     }
 
     export interface WriteFileCallback {
-        (fileName: string, data: string, writeByteOrderMark: boolean, onError?: (message: string) => void, sourceFiles?: ReadonlyArray<SourceFile>): void;
+        (fileName: string, data: string, writeByteOrderMark: boolean, onError: ((message: string) => void) | undefined, sourceFiles: ReadonlyArray<SourceFile>): void;
     }
 
     export class OperationCanceledException { }