Polymer element project templates: class name for Dart + source names based on generated tag

TBR: @umop

* The generated tag can differ from the project name, e.g. project name "MyElement" will produce a tag named "x-myelement". The output source names must or at least should match the tag, in this case `x-myelement.*` for Polymer JS and  `x_myelement.*` for Polymer Dart.
* The Polymer Dart element template uses `className`: generate that.