Fix noPadding attribute not working when set programmatically as opposed to statically

This worked:
  HTML: <spark-button noPadding></>
But this didn't:
  HTML: <spark-button id="btn"></>
  Dart: (querySelector('#btn') as SparkButton).noPadding = true;

The reason was the half-working support for data-binding in an inline <style> tag in a Polymer custom element's markup: when used around an inline <style> tag, <template if="{{noPadding}}"> worked for statically set noPadding, but failed to see programmatic changes to it, as described above.
Merge pull request #1879 from ussuri/fix-noPadding-in-spark-button

Fix spark-button: noPadding attribute didn't work when set programmatically (vs. statically)