A value can't be selected by clicking on IE11

[Tomi Virkki]

1. Open the first demo on IE11
2. Select a value by clicking a date on the dropdown
   -> Placeholder is shown, no value is visible

[Yuriy Yevstihnyeyev]

BTW, tests are broken on master in ie11 cause this.$.overlay is not available over here: 2d86da5#diff-0f7cab7f0a6b1e79bf518eb211b482deR328

[Serhii Kulykov]

Let's consider using disable-upgrade instead of dom-if as in vaadin/vaadin-combo-box#776