Reset declarative to false when calling attachShadow()

This was an oversight in the spec and this matches browser behavior. This makes a second call to `attachShadow()` throw.

Closes #1234.

dom.bs

@@ -6965,8 +6965,17 @@ a boolean <var>delegatesFocus</var>, and a boolean <var>slotAssignment</var>:
    is false, then <a>throw</a> an "{{NotSupportedError!!exception}}" {{DOMException}}.
 
    <li>
-    <p>Otherwise, <a for=/>remove</a> all of <var>element</var>'s <a for=Element>shadow root</a>'s
-    <a for=tree>children</a>, in <a>tree order</a>, and return.
+    <p>Otherwise:
+
+    <ol>
+     <li><p><a for=/>Remove</a> all of <var>element</var>'s <a for=Element>shadow root</a>'s
+     <a for=tree>children</a>, in <a>tree order</a>.
+
+     <li><p>Set <var>element</var>'s <a for=Element>shadow root</a>'s
+     <a for=ShadowRoot>declarative</a> to false.
+
+     <li><p>Return.
+    </ol>
 
     <p class=note>This means that if multiple declarative shadow roots are contained within a single
     shadow host, only the last one will remain.