Added iron-flext-layout-classes and included the iron-flext and iron-flex-alignment styles.

[arielmagbanua]

Added paper-styles as import. Without it the paper-header-panel or the paper-drawer-panel won't render properly, both panel won't occupy the entire top part of browser window and the content won't show.

[arielmagbanua]

@notwaldorf yeah I see the real issue why header panel is not rendering properly in the responsive material design layouts guides. Thanks!

[arielmagbanua]

@notwaldorf I pushed some new changes in this request. I Added iron-flext-layout-classes and included the iron-flext and iron-flex-alignment styles, it is needed by the header panels without it the header and content won't render properly and you can visibly see a gap space between the header and the top area of the browser window.

[notwaldorf]

/cc @arthurevans @blasten since this is docs and code i don't actually know about

[blasten]

I'd merge it if it looks good to @arthurevans

[arthurevans]

LGTM, thanks!