function requireMode(mode: string | CodeMirror.modespec): Promise<CodeMirror.modespec> {
  let modename = (typeof mode === 'string') ? mode :
      mode.mode || mode.name;
  let mimetype = (typeof mode !== 'string') ? mode.mime : '';
  let ext = modename.split('.').pop();

  // Get a modespec object by whatever means necessary.
  let info: CodeMirror.modespec = (
      (modename && mimetype && mode as CodeMirror.modespec) ||
      CodeMirror.findModeByName(modename) ||
      CodeMirror.findModeByExtension(ext) ||
      CodeMirror.findModeByMIME(modename) ||
      {mode: modename, mime: modename}
  );

  // Simplest, cheapest check by mode name.
  if (CodeMirror.modes.hasOwnProperty(modename)) {
    return Promise.resolve(info);
  }

  // Fetch the mode asynchronously.
  return new Promise<CodeMirror.modespec>((resolve, reject) => {
    require([`codemirror/mode/${info.mode}/${info.mode}`], () => {
      resolve(info);
    });
  });
}

function mimetypeForLangauge(info: IKernelLanguageInfo): string {
  // Use the codemirror mode if given since some kernels rely on it.
  let mode = info.codemirror_mode;
  let mime = 'text/plain';
  if (mode) {
    if (typeof mode === 'string') {
      if (CodeMirror.modes.hasOwnProperty(mode as string)) {
        mode = CodeMirror.modes[mode as string];
      } else {
        mode = CodeMirror.findModeByName(mode as string);
      }
    } else if ((mode as CodeMirror.modespec).mime) {
      // Do nothing.
    } else if ((mode as CodeMirror.modespec).name) {
      let name = (mode as CodeMirror.modespec).name;
      mode = CodeMirror.findModeByName(name);
    }
    if (mode) {
      mime = (mode as CodeMirror.modespec).mime;
    }
  } else if (info.mimetype) {
    mime = info.mimetype;
  }
  return mime;
}

/**
 * Load a CodeMirror mode based on a mode spec.
 */
function loadInfo(editor: CodeMirror.Editor, info: CodeMirror.modespec): void {
  if (!info) {
    editor.setOption('mode', 'null');
    return;
  }
  if (CodeMirror.modes.hasOwnProperty(info.mode)) {
    editor.setOption('mode', info.mime);
  } else {
    require([`codemirror/mode/${info.mode}/${info.mode}`], () => {
      editor.setOption('mode', info.mime);
    });
  }
}

function requireMode(mode: string | CodeMirror.modespec): Promise<CodeMirror.modespec> {
  let info = findMode(mode);

  // Simplest, cheapest check by mode name.
  if (CodeMirror.modes.hasOwnProperty(info.mode)) {
    return Promise.resolve(info);
  }

  // Fetch the mode asynchronously.
  return new Promise<CodeMirror.modespec>((resolve, reject) => {
    require([`codemirror/mode/${info.mode}/${info.mode}`], () => {
      resolve(info);
    });
  });
}

  function ensure(mode: string | ISpec): Promise<ISpec> {
    let spec = findBest(mode);

    // Simplest, cheapest check by mode name.
    if (CodeMirror.modes.hasOwnProperty(spec.mode)) {
      return Promise.resolve(spec);
    }

    // Fetch the mode asynchronously.
    return new Promise<ISpec>((resolve, reject) => {
      require([`codemirror/mode/${spec.mode}/${spec.mode}.js`], () => {
        resolve(spec);
      });
    });
  }

  export function ensure(mode: string | ISpec): Promise<ISpec> {
    let spec = findBest(mode);

    // Simplest, cheapest check by mode name.
    if (CodeMirror.modes.hasOwnProperty(spec.mode)) {
      return Promise.resolve(spec);
    }

    // Fetch the mode asynchronously.
    return new Promise<ISpec>((resolve, reject) => {
      // An arrow function below seems to miscompile in our current webpack to
      // invalid js.
      require([`codemirror/mode/${spec.mode}/${spec.mode}.js`], function() {
        resolve(spec);
      });
    });
  }

 loader: async spec => CodeMirror.modes.hasOwnProperty(spec.mode),