introduce web_components pkg to replace shadow_dom, html_import, custom_element. This new package is 100% compatible with the polyfills used by http://www.polymer-project.org/

Deletes:
  pkg/shadow_dom
  pkg/custom_element (except web_ui stuff)
  pkg/html_import

Adds:
  pkg/web_components, with README.md and build instructions

Changes:
  pkg/html5lib -- deprecate queryAll/query in favor of *Selector
  pkg/polymer -- test fixes & a few small changes to work with new polyfills, mostly in build
  tests/html/custom -- can't load the CE polyfill at runtime, must now be injected statically via an HTML page, similar to the style @blois used for some custom element tests.

R=blois@google.com, sigmund@google.com

Review URL: https://codereview.chromium.org//158083002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/web_components@32533 260f80e4-7a28-3924-810f-c04153c831b5