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 +6965,16 @@ 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.