Make _asyncClick easier to test, or provide a test helper

[mgiuffrida]

MockInteractions.pressAndReleaseKeyOn uses Polymer.Base.async to queue the keyUp event.

IronButtonState._asyncClick uses this.async to queue the click event, with a delay of 1.

This means test code has to do this:

MockInteractions.pressEnter(foo);
Polymer.dom.flush();  // flush MockInteraction's async call
foo.async(function() {
  // test foo
}, 1);  // schedule a setTimeout with a delay >= 1, so it runs after _asyncClick

The async call must have a delay greater than or equal to the one used in _asyncClick thanks to step 13. If you ever change the delay here, say from 1 to 10, my test code will fail.

(The Polymer.dom.flush() in the above is extraneous, but included for clarity.)

It would be nice if I could just do:

MockInteractions.pressEnter(foo);
Polymer.dom.flush();  // flush all the things
// test foo

or even:

MockInteractions.pressEnter(foo);
Polymer.dom.flush();  // flush MockInteraction's async call
Polymer.dom.flush();  // flush IronButtonState's async call
// test foo

so I don't have to hard-code constants and keep them up-to-date with _asyncClick.