Performance improvements for DOM creation
It now caches all created HTML fragments and uses .clone for fast instantiation

Also fix createShadowRoot to not put the class on there (that was an obsolete impl detail)
JacobR suggested using a DocumentFragment, I'll give that a try next.

Review URL: https://chromiumcodereview.appspot.com//12326143
remove deprecated features

Review URL: https://chromiumcodereview.appspot.com//12319106
reduce memory of LinkedList, by inlining the sentinel into LinkedList, which now has a _head and _tail. This makes .next and .previous simpler (no need to check for sentinel).

Review URL: https://chromiumcodereview.appspot.com//12263053
Fixes #370, support @observable in component's dart file when the file is external (not an inline script).

We need to track edits against the DartCodeInfo instead of the LibraryInfo.

Review URL: https://chromiumcodereview.appspot.com//12334110
Adding flags to dwc to define package-root and to not rewrite-urls

Review URL: https://chromiumcodereview.appspot.com//12376003
Release new version of webui.

Review URL: https://chromiumcodereview.appspot.com//12387005
Merge branch 'master' of https://github.com/dart-lang/web-ui into polyfill_default
bump version