Move the check on registering an element interface as a custom element

Previously, this would be prevented at definition time. However, that
required extra work to know all of the possible element interfaces, and
was inconsistent with other failure cases. This moves the necessary
check to construction time, which allows it to be simplified. Fixes
WICG/webcomponents#541.

source

@@ -2915,7 +2915,6 @@ a.setAttribute('href', 'http://example.com/'); // change the content attribute d
      <li><dfn data-x-href="https://heycam.github.io/webidl/#dfn-perform-a-security-check">perform a security check</dfn>
      <li><dfn data-x-href="https://heycam.github.io/webidl/#dfn-platform-object">platform object</dfn></li>
      <li><dfn data-x-href="https://heycam.github.io/webidl/#dfn-interface-object">interface object</dfn></li>
-     <li><dfn data-x-href="https://heycam.github.io/webidl/#dfn-inherited-interfaces">inherited interfaces</dfn></li>
      <li><dfn data-x-href="https://heycam.github.io/webidl/#es-platform-objects">global environment associated with</dfn> a platform object</li>
      <li><dfn data-noexport="" data-x-href="https://heycam.github.io/webidl/#dfn-callback-context">callback context</dfn>
      <li><dfn data-x-href="https://heycam.github.io/webidl/#dfn-frozen-array-type">frozen array</dfn>
@@ -9757,6 +9756,26 @@ interface <dfn>HTMLUnknownElement</dfn> : <span>HTMLElement</span> { };</pre>
    <li><p>Let <var>registry</var> be the <span>current global object</span>'s
    <code>CustomElementsRegistry</code> object.</p></li>
 
+   <li>
+    <p>If NewTarget is equal to the <span>active function object</span>, then throw a
+    <code>TypeError</code> and abort these steps.</p>
+
+    <div class="example no-backref">
+     <p>This can occur when a custom element is defined using an <span>element interface</span> as
+     its constructor:</p>
+
+     <pre>customElements.define("bad-1", HTMLButtonElement);
+new HTMLButtonElement();          // (1)
+document.createElement("bad-1");  // (2)</pre>
+
+     <p>In this case, during the execution of <code>HTMLButtonElement</code> (either explicitly, as
+     in (1), or implicitly, as in (2)), both the <span>active function object</span> and NewTarget
+     are <code>HTMLButtonElement</code>. If this check was not present, it would be possible to
+     create an instance of <code>HTMLButtonElement</code> whose local name was <code
+     data-x="">bad-1</code>.</p>
+    </div>
+   </li>
+
    <li>
     <p>Let <var>definition</var> be the entry in <var>registry</var> with <span
     data-x="concept-custom-element-definition-constructor">constructor</span> equal to NewTarget. If
@@ -9783,10 +9802,10 @@ interface <dfn>HTMLUnknownElement</dfn> : <span>HTMLElement</span> { };</pre>
        <p>This can occur when a custom element is defined to not extend any local names, but
        inherits from a non-<code>HTMLElement</code> class:</p>
 
-       <pre>customElements.define("bad-1", class Bad1 extends HTMLParagraphElement {});</pre>
+       <pre>customElements.define("bad-2", class Bad2 extends HTMLParagraphElement {});</pre>
 
        <p>In this case, during the (implicit) <code data-x="">super()</code> call that occurs when
-       constructing an instance of <code data-x="">Bad1</code>, the <span>active function
+       constructing an instance of <code data-x="">Bad2</code>, the <span>active function
        object</span> is <code>HTMLParagraphElement</code>, not <code>HTMLElement</code>.</p>
       </div>
      </li>
@@ -9811,10 +9830,10 @@ interface <dfn>HTMLUnknownElement</dfn> : <span>HTMLElement</span> { };</pre>
        <p>This can occur when a custom element is defined to extend a given local name but inherits
        from the wrong class:</p>
 
-       <pre>customElements.define("bad-2", class Bad2 extends HTMLQuoteElement {}, { extends: "p" });</pre>
+       <pre>customElements.define("bad-3", class Bad3 extends HTMLQuoteElement {}, { extends: "p" });</pre>
 
        <p>In this case, during the (implicit) <code data-x="">super()</code> call that occurs when
-       constructing an instance of <code data-x="">Bad2</code>, <var>valid local names</var> is the
+       constructing an instance of <code data-x="">Bad3</code>, <var>valid local names</var> is the
        list containing <code>q</code> and <code>blockquote</code>, but <var>definition</var>'s <span
        data-x="concept-custom-element-definition-local-name">local name</span> is <code>p</code>,
        which is not in that list.</p>
@@ -66411,32 +66430,6 @@ dictionary <dfn>ElementDefinitionOptions</dfn> {
    <li><p>If <span>IsConstructor</span>(<var>constructor</var>) is false, then throw a
    <code>TypeError</code> and abort these steps.</p></li>
 
-   <li>
-    <p>If <var>constructor</var> is either an <span>interface object</span> or <span>named
-    constructor</span>, whose corresponding interface either is <code>HTMLElement</code> or has
-    <code>HTMLElement</code> in its set of <span>inherited interfaces</span>, throw a
-    <code>TypeError</code> and abort these steps.</p>
-
-    <div class="note">
-     <p>This prevents passing, for example, <code class="no-backref">HTMLButtonElement</code> as
-     <var>constructor</var>, and thus enabling the creation of <code
-     class="no-backref">HTMLButtonElement</code> instances with a local name that is not <code
-     class="no-backref">button</code>.</p>
-
-     <p>Author-defined <span data-x="custom element constructor">custom element constructors</span>,
-     created for example via <code data-x="">class extends HTMLElement {}</code> or <code
-     data-x="">class extends HTMLButtonElement {}</code>, will pass this step without throwing a
-     <code>TypeError</code>, since they are not <span data-x="interface object">interface
-     objects</span>.</p>
-
-     <p><span data-x="interface object">Interface objects</span> that do not inherit from
-     <code>HTMLElement</code> (for example <code class="no-backref">TextTrack</code>), or classes
-     defined in the JavaScript specification (such as <code data-x="">Set</code>), will pass this
-     step without throwing a <code>TypeError</code>. However, they will later cause <span>create an
-     element</span> to fail.</p>
-    </div>
-   </li>
-
    <li><p>If <var>name</var> is not a <span>valid custom element name</span>, then throw a
    <span>"<code>SyntaxError</code>"</span> <code>DOMException</code> and abort these steps.</p></li>