warning by async loading in chrome

[s-trooper]

Hi, if i load this element with async attr like:

<link rel="import" href="/shared/polymer/bower_components/paper-icon-button/paper-icon-button.html" async>
<link rel="import" href="/shared/polymer/bower_components/paper-toolbar/paper-toolbar.html" async>

i get in chrome a warning but not in FF:

[paper-listbox::_flattenBehaviorsList]: behavior is null, check for missing or 404 import   polymer-micro.html:277 
[paper-item::_flattenBehaviorsList]: behavior is null, check for missing or 404 import    polymer-micro.html:277 
[paper-button::_flattenBehaviorsList]: behavior is null, check for missing or 404 import

Browsers Affected

• [x] Chrome
• [ ] Firefox
• [ ] Safari 9
• [ ] Safari 8
• [ ] Safari 7
• [ ] Edge
• [ ] IE 11
• [ ] IE 10

[IntranetFactory]

I think the root cause of this problem is https://github.com/Polymer/polymer/issues/2522