fix paper-input #531

[christophe-g]

Fixes (reported in PolymerElements/paper-input#531)

Thrown by paper-textarea with initial value and auto-validate like:

<paper-textarea label="autoresizing textarea input" value="will throw" auto-validate></paper-textarea>

In this case, validation happens too early causing a read of this.$.textarea.validity.valid to cause the error (this.$ is undefined)

[christophe-g]

Comment for reviewer - I branched from origin/2.x maybe causing the merge conflict. Changes are really minimal.

[e111077]

Hi, we are unfortunately going through a large internal migration of paper-input that may result in a new major release. I'll try to make sure this change gets into that change. That, coupled with this element's owner changing soon may unfortunately cause this to take a while. I recommend a fork if you need this in now.

[e111077]

Please include a test