Add customElements.whenDefined() API

As discussed in WICG/webcomponents#427, this
adds the ability for authors to wait for a given custom element name to
become defined.

source

@@ -66977,7 +66977,7 @@ console.log(plasticButton2.getAttribute("is")); // will output "plastic-button"<
    is truly one-time will need a guard to prevent it from running twice.</p></li>
 
    <li><p>In general, the constructor should be used to set up initial state and default values, and
-   to set up event listeners and possibly a shadow root</p>.</li>
+   to set up event listeners and possibly a shadow root.</p></li>
   </ul>
 
   <p>Several of these requirements are checked during <span data-x="create an element">element
@@ -67202,6 +67202,7 @@ console.log(plasticButton2.getAttribute("is")); // will output "plastic-button"<
   <pre class="idl">interface <dfn>CustomElementsRegistry</dfn> {
   void <span data-x="dom-CustomElementsRegistry-define">define</span>(DOMString name, Function constructor, optional ElementRegistrationOptions options);
   any <span data-x="dom-CustomElementsRegistry-get">get</span>(DOMString name);
+  Promise&lt;void&gt; <span data-x="dom-CustomElementsRegistry-whenDefined">whenDefined</span>(DOMString name);
 };
 
 dictionary <dfn>ElementRegistrationOptions</dfn> {
@@ -67215,6 +67216,11 @@ dictionary <dfn>ElementRegistrationOptions</dfn> {
   data-x="concept-custom-element-definition-local-name">local name</span>, or <span
   data-x="concept-custom-element-definition-constructor">constructor</span>.</p>
 
+  <p>Every <code>CustomElementsRegistry</code> also has a <dfn>when-defined promise map</dfn>,
+  mapping <span data-x="valid custom element name">valid custom element names</span> to lists of
+  promises. It is used to implement the <code
+  data-x="dom-CustomElementsRegistry-whenDefined">whenDefined()</code> method.</p>
+
   <dl class="domintro">
    <dt><var>window</var> . <code data-x="dom-window-customElements">customElements</code> . <code
    subdfn data-x="dom-CustomElementsRegistry-define">define</code>(<var>name</var>,
@@ -67240,6 +67246,14 @@ dictionary <dfn>ElementRegistrationOptions</dfn> {
    data-x="concept-custom-element-definition-name">name</span>. Returns undefined if there is no
    <span>custom element definition</span> with the given <span
    data-x="concept-custom-element-definition-name">name</span>.</dd>
+
+   <dt><var>window</var> . <code data-x="dom-window-customElements">customElements</code> . <code
+   subdfn data-x="dom-CustomElementsRegistry-whenDefined">whenDefined</code>(<var>name</var>)</dt>
+
+   <dd>Returns a promise that will be fulfilled when a <span>custom element</span> becomes defined
+   with the given name. (If such a <span>custom element</span> is already defined, the returned
+   promise will be immediately fulfilled.) Returns a promise rejected with a
+   <code>SyntaxError</code> if not given a <span>valid custom element name</span>.</dd>
   </dl>
 
   <p><dfn>Element definition</dfn> is a process of adding a <span>custom element definition</span>
@@ -67336,6 +67350,21 @@ dictionary <dfn>ElementRegistrationOptions</dfn> {
    <var>attributeChangedCallback</var> (stored by their corresponding name).</p></li>
 
    <li><p>Add <var>definition</var> to this <code>CustomElementsRegistry</code>.</p></li>
+
+   <li>
+    <p>If this <code>CustomElementsRegistry</code>'s <span>when-defined promise map</span>
+    contains an entry with key <var>name</var>:</p>
+
+    <ol>
+     <li><p>Let <var>promiseList</var> be the value of that entry.</p></li>
+
+     <li><p>For each <var>promise</var> in <var>promiseList</var>, resolve <var>promise</var> with
+     undefined.</p></li>
+
+     <li><p>Delete that entry from this <code>CustomElementsRegistry</code>'s <span>when-defined
+     promise map</span>.</p></li>
+    </ol>
+   </li>
   </ol>
 
   <p>When invoked, the <dfn><code
@@ -67350,6 +67379,58 @@ dictionary <dfn>ElementRegistrationOptions</dfn> {
    <li><p>Otherwise, return undefined.</p></li>
   </ol>
 
+  <p>When invoked, the <dfn><code
+  data-x="dom-CustomElementsRegistry-whenDefined">whenDefined(<var>name</var>)</code></dfn> method
+  must run these steps:</p>
+
+  <ol>
+   <li><p>If <var>name</var> is not a <span>valid custom element name</span>, then return a promise
+   rejected with a <code>SyntaxError</code> exception and abort these steps.</p></li>
+
+   <li><p>If this <code>CustomElementsRegistry</code> contains an entry with <span
+   data-x="concept-custom-element-definition-name">name</span> <var>name</var>, then return a
+   promise resolved with undefined and abort these steps.</p></li>
+
+   <li><p>Let <var>map</var> be this <code>CustomElementsRegistry</code>'s <span>when-defined
+   promise map</span>.</p></li>
+
+   <li><p>If <var>map</var> does not contain an entry with key <var>name</var>, create an entry in
+   <var>map</var> with key <var>name</var> and whose value is an empty list.</p></li>
+
+   <li><p>Let <var>promiseList</var> be the value of the entry in <var>map</var> with key
+   <var>name</var>.</p></li>
+
+   <li><p>Let <var>promise</var> be a new promise, and append <var>promise</var> to
+   <var>promiseList</var>.</p></li>
+
+   <li><p>Return <var>promise</var>.</p></li>
+  </ol>
+
+  <div class="example">
+   <p>The <code data-x="dom-CustomElementsRegistry-whenDefined">whenDefined()</code> method can be
+   used to avoid performing an action until all appropriate <span data-x="custom element">custom
+   elements</span> are <span data-x="concept-element-defined">defined</span>. In this example, we
+   combine it with the <code data-x="selector-defined">:defined</code> pseudo-class to hide a
+   dynamically-loaded article's contents until we're sure that all of the <span data-x="autonomous
+   custom element">autonomous custom elements</span> it uses are defined.</p>
+
+   <pre>articleContainer.hidden = true;
+
+fetch(articleURL)
+  .then(response => response.text())
+  .then(text => {
+    articleContainer.innerHTML = text;
+
+    return Promise.all(
+      [...articleContainer.querySelectorAll(":not(:defined)")]
+        .map(el => customElements.whenDefined(el.localName))
+    );
+  })
+  .then(() => {
+    articleContainer.hidden = false;
+  });</pre>
+  </div>
+
   <h4><dfn data-x="custom-element-upgrades">Upgrades</dfn></h4>
 
   <p>To <dfn data-x="concept-upgrade-an-element" data-export="">upgrade an element</dfn>, given as
@@ -119286,6 +119367,7 @@ INSERT INTERFACES HERE
   J. King,
   Jacob Davies,
   Jacques Distler,
+  Jake Archibald,
   Jake Verbaten,
   Jakub &#321;opusza&#324;ski,
   Jakub Wilk,