Polymer.dom.flush() only if Polymer exists

[usergenic]

From #611. This PR is just to ensure passing tests on latest rebase.

Looks like we essentially do the same in https://github.com/Polymer/tools/blob/master/packages/web-component-tester/browser/environment/helpers.ts#L128 but more so...

  let scope;
  if (window.Polymer && window.Polymer.dom && window.Polymer.dom.flush) {
    scope = window.Polymer.dom;
  } else if (window.Polymer && window.Polymer.flush) {
    scope = window.Polymer;
  } else if (window.WebComponents && window.WebComponents.flush) {
    scope = window.WebComponents;
  }
  if (scope) {
    scope.flush();
  }

I won't amend a11ysuite.js to do that yet, but it is worth noting.

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