This has the following implications:

 - There's no createElement() function any more. If you want to create
   an element from script, you have to use its constructor.

 - There's no async element registration. The parser will block until
   all the imports are imported when you use a tag name of a custom
   element that hasn't been registered yet, in case one of the imports
   defines it.

 - If you try to construct a non-registered element in markup, it
   turns into an <error> element.

 - <div>, <span>, and <error> are new built-in elements.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/695423004