Add customElements.get() to retrieve a custom element constructor

Fixes WICG/webcomponents#445.

source

@@ -67168,6 +67168,7 @@ 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);
 };
 
 dictionary <dfn>ElementRegistrationOptions</dfn> {
@@ -67198,6 +67199,14 @@ dictionary <dfn>ElementRegistrationOptions</dfn> {
    a <span>customized built-in element</span> for the <span>element type</span> identified by the supplied
    <var>baseLocalName</var>. A <code>NotSupportedError</code> will be thrown upon trying to extend a
    <span>custom element</span> or an unknown element.</dd>
+
+   <dt><var>window</var> . <code data-x="dom-window-customElements">customElements</code> . <code
+   data-x="dom-CustomElementsRegistry-get">get</code>(<var>name</var>)</dt>
+
+   <dd>Retrieves the <span>custom element constructor</span> defined for the given <span
+   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>
   </dl>
 
   <p><dfn>Element definition</dfn> is a process of adding a <span>custom element definition</span>
@@ -67296,6 +67305,18 @@ dictionary <dfn>ElementRegistrationOptions</dfn> {
    <li><p>Add <var>definition</var> to this <code>CustomElementsRegistry</code>.</p></li>
   </ol>
 
+  <p>When invoked, the <dfn><code
+  data-x="dom-CustomElementsRegistry-get">get(<var>name</var>)</code></dfn> method must run these
+  steps:</p>
+
+  <ol>
+   <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 that
+   entry's <span data-x="concept-custom-element-definition-constructor">constructor</span>.</p></li>
+
+   <li><p>Otherwise, return undefined.</p></li>
+  </ol>
+
   <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