[WIP] Temporal proposal HTML serialization support

This patch lets Temporal objects be serialized, both in storage and with
postMessage, if they use built-in calendars and timezones. This is the
only expected integration of Temporal with HTML/DOM expected initially;
see earlier discussion in w3ctag/design-reviews#311

The current draft just includes Temporal.PlainDate, and it will be extended
to all of the Temporal types after initial review. This patch should not be
landed until (at least) Temporal reaches Stage 3.

source

@@ -2876,6 +2876,15 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
     <p>User agents that support JavaScript must also implement the <cite>Top-Level Await</cite>
     proposal. <ref spec=JSTLA></p>
 
+    <p>User agents that support JavaScript must also implement the <cite>Temporal</cite>
+    proposal. The following terms are defined in the Temporal proposal and used in this
+    specification: <ref spec=JSTEMPORAL></p>
+
+    <ul>
+     <li>The <dfn data-x-href="https://tc39.es/proposal-temporal/#sec-temporal.plaindate">Temporal.PlainDate</dfn> constructor</li>
+    </ul>
+   </dd>
+
     <p>User agents that support JavaScript must also implement the <cite>Import Assertions</cite>
     proposal. The following terms are defined there, and used in this specification: <ref
     spec=JSIMPORTASSERTIONS></p>
@@ -8422,6 +8431,21 @@ interface <dfn interface>DOMStringList</dfn> {
     </ol>
    </li>
 
+   <li>
+    <p>Otherwise, if <var>value</var> has an [[InitializedTemporalDate]] internal slot, then:</p>
+
+    <ol>
+     <li><p>If <var>value</var>.[[Calendar]] does not have a [[InitializedTemporalCalendar]]
+     internal slot, then throw a <span>"<code>DataCloneError</code>"</span>
+     <code>DOMException</code>.</p></li>
+
+     <li><p>Set <var>serialized</var> to { [[Type]]: "Temporal.PlainDate", [[ISOYear]]:
+     <var>value</var>.[[ISOYear]], [[ISOMonth]]: <var>value</var>.[[ISOMonth]], [[ISODay]]:
+     <var>value</var>.[[ISODay]], [[Calendar]]: <var>value</var>.[[Calendar]].[[Identifier]]
+     }.</p></li>
+    </ol>
+   </li>
+
    <li>
     <p>Otherwise, if <var>value</var> is a <span>platform object</span> that is a <span
     data-x="serializable objects">serializable object</span>:</p>
@@ -8831,6 +8855,17 @@ o.myself = o;</code></pre>
     </ol>
    </li>
 
+   <li>
+    <p>Otherwise, if <var>serialized</var>.[[Type]] is "Temporal.PlainDate", then:</p>
+
+    <ol>
+     <li><p>Set <var>value</var> to ! <span>Construct</span>(<span
+     data-x="Temporal.PlainDate">%Temporal.PlainDate%</span>, &laquo;
+     <var>serialized</var>.[[ISOYear]], <var>serialized</var>.[[ISOMonth]],
+     <var>serialized</var>.[[ISODay]], &raquo;).</p></li>
+    </ol>
+   </li>
+
    <li>
     <p>Otherwise:</p>
 
@@ -125485,6 +125520,9 @@ INSERT INTERFACES HERE
    <dt id="refsJSTLA">[JSTLA]</dt>
    <dd><cite><a href="https://tc39.es/proposal-top-level-await/">Top-Level Await</a></cite>. Ecma International.</dd>
 
+   <dt id="refsJSTEMPORAL">[JSTTEMPORAL]</dt>
+   <dd><cite><a href="https://tc39.es/proposal-temporal/">Temporal</a></cite>. Ecma International.</dd>
+
    <dt id="refsJSON">[JSON]</dt>
    <dd><cite><a href="https://tools.ietf.org/html/rfc8259">The JavaScript Object Notation (JSON) Data Interchange Format</a></cite>, T. Bray. IETF.</dd>