Support for constant attributes passed to web components

Review URL: https://chromiumcodereview.appspot.com//11708002
Switch to use browser/dart.js (fix #300)

Review URL: https://chromiumcodereview.appspot.com//12079051
Update CHANGELOG.md
Push a new version with the latest features (includes printing warnings again)

Review URL: https://chromiumcodereview.appspot.com//12084070
fix for breaking html5lib change

Review URL: https://chromiumcodereview.appspot.com//12087111
fix #304 - better warnings for missing script type

Review URL: https://chromiumcodereview.appspot.com//11878012
Added CSS polyfill

Mutation problem

minor cleanup

make CSS polyfill optional --css_polyfill

changes to match updated csslib.

integrate CR comments

comment cleanup

cleaned a few func

cleanup

integrated CL comments
integrated CL comments