Deprecate old flexbox syntax used by IE 10 to make Lighthouse happy

[web-padawan]

Description

As of now, Polymer 2.0 is unlikely to support IE 10, due to Mutation Observer bugs and more. So, shall we still include old 2012 syntax prefixes?

Lighthouse recommends not to use it anymore. What about having a breaking change here? Or should this be considered as a future change for 3.x element release, which should come with switching from hybrid elements to 2.0 syntax?

[notwaldorf]

If we don't support IE10, we probably don't need any of iron-flex-layout, right? Are there any browsers that don't support the generic, non-prefixed styles?

[web-padawan]

AFAIK, Polymer 2 should still support Safari 8, which supports flexbox with -webkit prefixes.
And looks like Lighthouse should not warn about these.