message: Simplify the FileMapUrlLoader.
diff: @@ -44,10 +44,10 @@ export class FileMapUrlLoader implements UrlLoader {
  async load(url: string): Promise<string> {
  const file = this.files.get(this._filePath(url))!;
  - if (file) {
  - return Promise.resolve(file.contents.toString());
  - } else {
  - return Promise.reject(undefined);
  + if (file == null) {
  + throw new Error(`File ${url} not present in file map.`);
  }
  +
  + return file.contents.toString();
  }
  }
  