How to use the @jupyterlab/codemirror.Mode.addSpecLoader function in @jupyterlab/codemirror

it('should add a spec loader', async () => {
      let called = 0;
      let loaded = 0;

      Mode.addSpecLoader(async spec => {
        called++;
        if (spec.mode !== 'bar') {
          return false;
        }
        loaded++;
        return true;
      }, 42);

      CodeMirror.modeInfo.push(fakeMode('bar'));

      let spec = await Mode.ensure('bar');
      expect(called).to.equal(1);
      expect(loaded).to.equal(1);
      expect(spec!.name).to.equal('BAR');

      spec = await Mode.ensure('python');