Remove unintentional minimum size of 2px for spark-splitter

This was due to border set to be 1px on both sides irrespective to the requested splitter size. It turns out that even with `box-sizing: border-box` the 2 borders aren't merged or eliminated when the requested element's size is 1px or 0px. This commit fixes that via additional CSS rules.
Revert accidental change
Fix dart2js warning
Clean up whitespace
Merge pull request #3484 from keertip/tavern

rev up tavern version
Add default value for enable-polymer-designer to app.json
Merge remote-tracking branch 'upstream/master' into cde-polymer-designer
Merge pull request #3487 from ussuri/cde-polymer-designer

[Polymer Designer] Add default for flag + eliminate dart2js warning + whitespace/comments
Bump current app version to match the official one
Merge pull request #3488 from ussuri/bump-tott-app-version

Bump current app's version in manifest to match the official one
Manually merge forgotten PD-related manifest changes
Merge pull request #3490 from ussuri/cde-polymer-designer

Manually merge forgotten Polymer Designer-related manifest changes
Merge pull request #3480 from ussuri/splitter-look-improvement

Remove unintentional minimum size of 2px for spark-splitter
fix for the outline view button
Merge pull request #3492 from devoncarew/devoncarew_refix_outline

fix for the outline view button
Created PreferenceContentProvider and tests.  Write will cause an onChange event to occur.
Releasing 0.16
Review fix and test update.
Merge pull request #3496 from dart-lang/umop-preferences-PreferenceStore_provider

Created PreferenceContentProvider