update: Polymer >=1.0.x
shady DOM
new PolymerDom(this).parentNode;
or
domHost
short for
Polymer.dom(this).getOwnerRoot().host
full shadow DOM
(this.parentNode as ShadowRoot).host
@ChristopheHerreman and @MattB are still right about encapsulation should not be broken.
But also JS Polymer elements access the parent in their layout elements because it's still convenient in some scenarios.
This works now in PolymerDart too.
Polymer.dart <= 0.16.x
(this.parentNode as ShadowRoot).host