Lazy-loaded <style is="custom-style"> containing mixin not shimmed correctly

[kevinpschaaf]

Description

Fix for #3705 left one failing test on Firefox which has native CSS properties but not HTML imports. In this configuration when useNativeCSSProperties is used, the following test fails (not caught due to intermittent selenium failures on CI):
https://github.com/Polymer/polymer/blob/master/test/runner.html#L75

Failure happens because lazy-loaded import containing script that calls Polymer({...}) causes synchronous registration & upgrade of an already-parsed element before a <custom-style> it depends on upgrades. This violates the timing requirements of the useNativeCSSProperties optimization, which requires that custom property bags be defined before they're used.

Live Demo / Steps to Reproduce

Run test highlighted below on Firefox:
https://github.com/Polymer/polymer/blob/master/test/runner.html#L75

Browsers Affected

• Chrome
• Firefox
• Edge
• Safari 9
• Safari 8
• IE 11

Versions

• Polymer: v1.5.0+master