currentScript needs to point to removed script elements

To avoid leaking shadow trees we added an “in a document” check in
caf203f. This turned out to not be
web compatible. So instead, we check whether the script element is in
a shadow tree. Ideally that is novel enough to not cause any
compatibility issues.

Fixes #1161.

source

@@ -3100,6 +3100,7 @@ a.setAttribute('href', 'http://example.com/'); // change the content attribute d
 
      <li><dfn data-noexport="" data-x-href="https://dom.spec.whatwg.org/#concept-node-document">node document</dfn> concept</li>
      <li><dfn data-noexport="" data-x="concept-DocumentFragment-host" data-x-href="https://dom.spec.whatwg.org/#concept-documentfragment-host">host</dfn> concept</li>
+     <li><dfn data-noexport="" data-x-href="https://dom.spec.whatwg.org/#concept-shadow-root">shadow root</dfn> concept</li>
 
      <li><dfn data-noexport="" data-x-href="https://dom.spec.whatwg.org/#interface-htmlcollection"><code>HTMLCollection</code></dfn> interface</li>
      <li><dfn data-noexport="" data-x="dom-HTMLCollection-length" data-x-href="https://dom.spec.whatwg.org/#dom-htmlcollection-length"><code>HTMLCollection.length</code></dfn> attribute</li>
@@ -59122,10 +59123,17 @@ o............A....e
      <dt>"<code data-x="">classic</code>"</dt>
      <dd>
       <ol>
-       <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. Otherwise, set it to null.</p></li>
+       <li>
+        <p>If the <code>script</code> element's <span>root</span> is <em>not</em> a <span>shadow
+        root</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. Otherwise, set it to null.</p>
+
+        <p class="note">This does not use the <span>in a document</span> check, as the
+        <code>script</code> element could have been removed from the document prior to execution,
+        and in that scenario <code data-x="dom-document-currentScript">currentScript</code> still
+        needs to point to it.</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>
@@ -67081,7 +67089,7 @@ console.log(plasticButton2.getAttribute("is")); // will output "plastic-button"<
    is truly one-time will need a guard to prevent it from running twice.</p></li>
 
    <li><p>In general, the constructor should be used to set up initial state and default values, and
-   to set up event listeners and possibly a shadow root.</p></li>
+   to set up event listeners and possibly a <span>shadow root</span>.</p></li>
   </ul>
 
   <p>Several of these requirements are checked during <span data-x="create an element">element