Initial version of spark-dialog

More-or-less works but is badly stiled (turned off Bootstrap)
Initial version of spark-dialog-button
Change default style to flat
Standardize overlay toggling attribute name ('overlay-toggle' -> 'overlayToggle')

Didn't change the usage in spark_polymer_ui.html, since that's going to be all rewritten.
set chrome 35 to be our min version
Don't wait for syncFs to load.
Merge pull request #2174 from dart-lang/fix_sync

Don't wait for syncFs to load.
Remove Bootstrap imports spark_polymer.css/spark_polymer_ui.css - imported through spark_common.css
Remove Bootstrap import from spark-menu - it's not using it
Merge pull request #2177 from ussuri/get-rid-of-bootstrap

Get rid of Bootstrap imports in a couple of places
Merge pull request #2164 from devoncarew/devoncarew_chrome_35

set chrome 35 to be our min version
Undo debugging artifacts
Address review comments
Correctly distribute main body content and spark-dialog-buttons + justify buttons to right
Merge pull request #2176 from ussuri/spark-dialog-2

Introducing <spark-dialog>
Adding entry points for examples
add path to spark-button
Merge pull request #2184 from financeCoding/fix_spark_widgets_examples

Fix spark widgets examples