Polymer Designer: switch to JS to workaround <webview>'s problem with display:none

@rginda 

This makes PD work in the deployed code. dart2js is missing support for `<webview>`: that's what's broken PD in Dart.

Also, this PR maked the PD dialog dynamically sized to take as much space as is available in the CDE window (it uses the now-fixed spark-dialog for that), and adds more actions to the dialog.
Better to check arguments.length here.  If the caller explicitly passes an undefined value, they should get some kind of error rather than have undefined silently coerced into a different value.