Add "disable shadow" flag check to element.attachShadow()

This was discussed as part of WICG/webcomponents#758, and integrates with whatwg/html#4324.

dom.bs

@@ -6767,6 +6767,23 @@ invoked, must run these steps:
  "<code>section</code>", or
  "<code>span</code>", then <a>throw</a> a "{{NotSupportedError!!exception}}" {{DOMException}}.
 
+ <li>
+  <p>If <a>context object</a>'s <a for=Element>local name</a> is a
+  <a>valid custom element name</a>, or <a>context object</a>'s
+  <a for=Element><code>is</code> value</a> is not null, then:
+
+  <ol>
+   <li><p>Let <var>definition</var> be the result of
+   <a lt="look up a custom element definition">looking up a custom element definition</a> given
+   <a>context object</a>'s <a for=Node>node document</a>, its <a for=Element>namespace</a>, its
+   <a for=Element>local name</a>, and its <a for=Element><code>is</code> value</a>.
+
+   <li><p>If <var>definition</var> is not null and <var>definition</var>'s
+   <a for="custom element definition">disable shadow</a> is true, then <a>throw</a> a
+   "{{NotSupportedError!!exception}}" {{DOMException}}.
+  </ol>
+ </li>
+
  <li><p>If <a>context object</a> is a <a for=Element>shadow host</a>, then <a>throw</a> an
  "{{NotSupportedError!!exception}}" {{DOMException}}.
 
@@ -10034,6 +10051,7 @@ J. S. Choi,
 Jungkee Song,
 Justin Summerlin,
 呂康豪 (Kang-Hao Lu),
+田村健人 (Kent TAMURA),
 Kevin Sweeney,
 Kirill Topolyan,
 Koji Ishii,