fix(@angular-devkit/core): NodeJSAsyncHost.isFile should check for a file

Author: clydin

packages/angular_devkit/core/node/host.ts

@@ -181,7 +181,7 @@ export class NodeJsAsyncHost implements virtualFs.Host<fs.Stats> {
   }
   isFile(path: Path): Observable<boolean> {
     return _callFs(fs.stat, getSystemPath(path)).pipe(
-      map(stat => stat.isDirectory()),
+      map(stat => stat.isFile()),
     );
   }