Clear the custom element reaction queue if upgrading fails

Fixes WICG/webcomponents#563, and an analogous
issue when the constructor uses return-override, by ensuring that all
exceptions go through the same path, which both sets the custom element
state to "failed", and clears the custom element reaction queue.

source

@@ -66692,62 +66692,64 @@ customElements.define("x-foo", class extends HTMLElement {
    data-x="concept-custom-element-definition-constructor">constructor</span>.</p></li>
 
    <li>
-    <p>Let <var>constructResult</var> be <span>Construct</span>(<var>C</var>).</p>
+    <p>Run the following substeps while catching any exceptions:</p>
 
-    <p class="note">If <var>C</var> <a href="#custom-element-conformance">non-conformantly</a> uses
-    an API decorated with the <code data-x="CEReactions">[CEReactions]</code> extended attribute,
-    then the reactions enqueued at the beginning of this algorithm will execute during this step,
-    before <var>C</var> finishes and control returns to this algorithm. Otherwise, they will execute
-    after <var>C</var> and the rest of the upgrade process finishes.</p>
-   </li>
+    <ol>
+     <li>
+      <p>Let <var>constructResult</var> be <span>Construct</span>(<var>C</var>).</p>
 
-   <li>
-    <p>Remove the last entry from the end of <var>definition</var>'s <span
-    data-x="concept-custom-element-definition-construction-stack">construction stack</span>.</p>
+      <p class="note">If <var>C</var> <a href="#custom-element-conformance">non-conformantly</a>
+      uses an API decorated with the <code data-x="CEReactions">[CEReactions]</code> extended
+      attribute, then the reactions enqueued at the beginning of this algorithm will execute during
+      this step, before <var>C</var> finishes and control returns to this algorithm. Otherwise, they
+      will execute after <var>C</var> and the rest of the upgrade process finishes.</p>
+     </li>
 
-    <div class="note">
-     <p>Assuming <var>C</var> calls <code data-x="">super()</code> (as it will if it is <a
-     href="#custom-element-conformance">conformant</a>), and that the call succeeds, this will be
-     the <span data-x="concept-already-constructed-marker"><i>already
-     constructed</i> marker</span> that replaced the <var>element</var> we pushed at the beginning
-     of this algorithm. (The <a href="#html-element-constructors">HTML element constructor</a>
-     carries out this replacement.)</p>
-
-     <p>If <var>C</var> does not call <code data-x="">super()</code> (i.e. it is not <a
-     href="#custom-element-conformance">conformant</a>), or if any step in the <a
-     href="#html-element-constructors">HTML element constructor</a> throws, then this entry will
-     still be <var>element</var>.</p>
-    </div>
-   </li>
+     <li>
+      <p>If <span>SameValue</span>(<var>constructResult</var>, <var>element</var>) is false,
+      then throw an <span>"<code>InvalidStateError</code>"</span> <code>DOMException</code>.</p>
 
-   <li>
-    <p>If <var>constructResult</var> is an abrupt completion, then:</p>
+      <p class="note">This can occur if <var>C</var> constructs another instance of the same custom
+      element before calling <code data-x="">super()</code>, or if <var>C</var> uses JavaScript's
+      <code data-x="">return</code>-override feature to return an arbitrary object from the
+      constructor.</p>
+     </li>
+    </ol>
+
+    <p>Then, perform the following substep, regardless of whether the above steps threw an exception
+    or not:</p>
 
     <ol>
-     <li><p>Set <var>element</var>'s <span>custom element state</span> to "<code
-     data-x="">failed</code>".</p></li>
+     <li>
+      <p>Remove the last entry from the end of <var>definition</var>'s <span
+      data-x="concept-custom-element-definition-construction-stack">construction stack</span>.</p>
 
-     <li><p>Return <var>constructResult</var> (i.e., rethrow the exception), and terminate these
-     steps.</p></li>
+      <div class="note">
+       <p>Assuming <var>C</var> calls <code data-x="">super()</code> (as it will if it is <a
+       href="#custom-element-conformance">conformant</a>), and that the call succeeds, this will be
+       the <span data-x="concept-already-constructed-marker"><i>already
+       constructed</i> marker</span> that replaced the <var>element</var> we pushed at the beginning
+       of this algorithm. (The <a href="#html-element-constructors">HTML element constructor</a>
+       carries out this replacement.)</p>
+
+       <p>If <var>C</var> does not call <code data-x="">super()</code> (i.e. it is not <a
+       href="#custom-element-conformance">conformant</a>), or if any step in the <a
+       href="#html-element-constructors">HTML element constructor</a> throws, then this entry will
+       still be <var>element</var>.</p>
+      </div>
+     </li>
     </ol>
-   </li>
 
-   <li>
-    <p>If <span>SameValue</span>(<var>constructResult</var>.[[value]], <var>element</var>) is false,
-    then:</p>
+    <p>Finally, if the above steps threw an exception, then:</p>
 
     <ol>
      <li><p>Set <var>element</var>'s <span>custom element state</span> to "<code
      data-x="">failed</code>".</p></li>
 
-     <li><p>Throw an <span>"<code>InvalidStateError</code>"</span> <code>DOMException</code>, and
-     terminate these steps.</p></li>
-    </ol>
+     <li><p>Empty <var>element</var>'s <span>custom element reaction queue</span>.</p>
 
-    <p class="note">This can occur if <var>C</var> constructs another instance of the same custom
-    element before calling <code data-x="">super()</code>, or if <var>C</var> uses JavaScript's
-    <code data-x="">return</code>-override feature to return an arbitrary object from the
-    constructor.</p>
+     <li><p>Rethrow the exception, and terminate this algorithm.</p></li>
+    </ol>
    </li>
 
    <li><p>Set <var>element</var>'s <span>custom element state</span> to "<code