Changes related to  issue 593989  have made some tests flaky or fragile, which a LOT of CLs in progress have encountered today.

Symptoms: "Script error." or "Uncaught TypeError: Cannot read property '$$' of null" in MD Settings browser tests.

Cause: in settings_main.js currentRouteChanged_ we schedule an async call to overscrollHeight_(). overscrollHeight_() inspects a bunch of elements that live in dom-ifs, so it returns early if the currentRoute property shows those elements won't exist.

However, because other changes to currentRoute can be made in between one currentRouteChanged_ call and the async call to overscrollHeight_(), using async() in currentRouteChanged_ is not sufficient to ensure that the dom-ifs reflect the **current** value of currentRoute while executing overscrollHeight_().

Possible fixes:

* Use setTimeout so other async(fn) operations and dom-if stamping are guaranteed to happen together, before entering overscrollHeight_()
* Call Polymer.dom.flush in overscrollHeight_() after checking currentRoute, to ensure the dom-ifs all accurately reflect currentRoute