build.py missing core-focusable

[mookiejones]

Need to fix Core-Focusable

The window wont show correctly because it is missing items from core-focusable.

• core-focusable.js
• polymer-mixin.js

execCmd('cp bower_components/core-focusable/core-focusable.js build/bower_components/core-focusable/core-focusable.js',
  silentFail=False)
execCmd('cp bower_components/core-focusable/polymer-mixin.js build/bower_components/core-focusable/polymer-mixin.js',
  silentFail=False)

fixes it.

Code also needed to be created to create the folders as well.

[sachinhosmani]

Thanks for filing this. Can you make a pull request?