Specs: Drop the concept of late-bound elements. An element can never be upgraded.

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

examples/home.sky

 #!mojo mojo:sky
-<sky>
+<div><!-- remove this element once Document can have multiple element children -->
 <style>
- h1 { font-size: 2em; margin: 1em; }
- p { margin: 0.5em 1em; color: #bcd8f5; font-weight: 900; }
+ .h1 { font-size: 2em; margin: 1em; }
+ .p { margin: 0.5em 1em; color: #bcd8f5; font-weight: 900; }
 </style>
-<h1>about:blank</h1>
-<p>Welcome to Sky!</p>
-</sky>
+<div class="h1">about:blank</div>
+<div class="p">Welcome to Sky!</div>
+</div>