Things to watch out for in the Polymer 2 upgrade

Sass

Nesting will not work with the :host or :slotted pseudo-classes.

This won't work:

:host {
  &[full-bleed] .content {
    padding: 0;
  }
}

Do this instead:

:host([full-bleed]) .content {
  padding: 0;
}

Import Paths

If you see 404s when loading local assets in Polymer 2.x:

<img src="[[importPath]]monogram-wdmk.png" see Polymer docs