list performance test

test.js

suite('performance issue', () => {
  test(
      'removing a list should not call layout more than once', async () => {
        let count = 0;
        const originalLayout = List.prototype.layout;
        List.prototype.layout = function(update) {
          originalLayout.call(this, update);
          count++;
        };

        const itemsTemplates = new Array(100).fill(0).map(() => listItem());
        fixt = await fixture(listTemplate({items: itemsTemplates}));
        element = fixt.root.querySelector('mwc-list')!;

        count = 0;

        fixt.remove();
        await element.updateComplete;
        fixt = null;
        assert.equal(
            count,
            1,
            'list.layout ran more than once while it shouldn\'t have');
        List.prototype.layout = originalLayout;
      });
});