Execute <script> in shadow trees

This makes <script> elements work when used in shadow trees.

Note that document.currentScript is set to null while running a <script>
in a shadow tree; see #1013 for details.

This takes care of most of #762, but it remains to make the "load" event
scoped, so we'll leave that issue open for now.

source

@@ -2166,6 +2166,12 @@ a.setAttribute('href', 'http://example.com/'); // change the content attribute d
   data-x="concept-node-remove-ext">removing steps</span> are invoked with it as argument and it is
   now no longer <span>in a document</span>.</p>
 
+  <p>A <dfn>node is connected</dfn> when the <span data-x="concept-node-insert-ext">insertion
+  steps</span> are invoked with it as argument and it is now <span>in a shadow-including
+  document</span>. Analogously, a <dfn>node is disconnected</dfn> when the <span
+  data-x="concept-node-remove-ext">removing steps</span> are invoked with it as argument and it is
+  now no longer <span>in a shadow-including document</span>.</p>
+
 
   <h4>Scripting</h4>
 
@@ -58540,23 +58546,21 @@ o............A....e
   <hr>
 
   <p>When a <code>script</code> element that is not marked as being <span>"parser-inserted"</span>
-  experiences one of the events listed in the following list, the user agent must <span>immediately</span>
-  <span data-x="prepare a script">prepare</span> the <code>script</code> element:</p>
+  experiences one of the events listed in the following list, the user agent must
+  <span>immediately</span> <span data-x="prepare a script">prepare</span> the <code>script</code>
+  element:</p>
 
   <ul>
 
-   <li>The <code>script</code> element gets <span data-x="node is inserted into a document">inserted
-   into a document</span>, at the time the <span data-x="nodes are inserted">node is inserted</span>
-   according to the DOM, after any other <code>script</code> elements inserted at the same time that
-   are earlier in the <code>Document</code> in <span>tree order</span>.</li>
+   <li>The <code>script</code> element gets <span data-x="node is connected">connected</span>.</li>
 
-   <li>The <code>script</code> element is <span>in a <code>Document</code></span> and a node or
+   <li>The <code>script</code> element is <span>in a shadow-including document</span> and a node or
    document fragment is <span data-x="nodes are inserted">inserted</span> into the
    <code>script</code> element, after any <code>script</code> elements <span data-x="nodes are
    inserted">inserted</span> at that time.</li>
 
-   <li>The <code>script</code> element is <span>in a <code>Document</code></span> and has a <code
-   data-x="attr-script-src">src</code> attribute set where previously the element had no such
+   <li>The <code>script</code> element is <span>in a shadow-including document</span> and has a
+   <code data-x="attr-script-src">src</code> attribute set where previously the element had no such
    attribute.</li>
 
   </ul>
@@ -59128,9 +59132,10 @@ o............A....e
      <dt>"<code data-x="">classic</code>"</dt>
      <dd>
       <ol>
-       <li><p>Set the <code>script</code> element's <span>node document</span>'s <code
+       <li><p>If the <code>script</code> element is <span>in a document</span>, then set the
+       <code>script</code> element's <span>node document</span>'s <code
        data-x="dom-document-currentScript">currentScript</code> attribute to the
-       <code>script</code> element.</p></li>
+       <code>script</code> element. Otherwise, set it to null.</p></li>
 
        <li><p><span data-x="run a classic script">Run the classic script</span> given by <span
        data-x="concept-script-script">the script's script</span>.</p></li>
@@ -59174,6 +59179,7 @@ o............A....e
 
     <p>Otherwise <span>queue a task</span> to <span>fire a simple event</span> named <code
     data-x="event-load">load</code> at the <code>script</code> element.</p>
+    <!-- Shadow TODO: this event needs to be scoped -->
 
    </li>