Normative: Permit serialization of BigInt wrappers

The goal of this PR is to integrate BigInt with HTML serialization.

This patch
- Adds BigInt to the "safe list" of primitives permitted for
  serialization.
- Adds serialization of BigInt wrappers, analogous to other wrappers.

Note that BigInt, like (the unserializable) Symbol does not have a
new-able constructor: use of wrappers is explicitly discouraged by
the specification. Nevertheless, this patch adds serialization support
for consistency with other wrappers.

web-platform-tests against postMessage on BigInt are out for review in
web-platform-tests/wpt#9565

source

@@ -3082,6 +3082,15 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
      <li>The <dfn data-x="js-HostGetImportMetaProperties" data-x-href="https://tc39.github.io/proposal-import-meta/#sec-hostgetimportmetaproperties">HostGetImportMetaProperties</dfn> abstract operation</li>
     </ul>
 
+    <p>Users agents that support JavaScript must also implement the <cite>BigInt</cite>
+    proposal. The following terms are defined there, and used in this specification: <ref
+    spec=JSBIGINT></p>
+
+    <ul class="brief">
+     <li>The <dfn data-x="js-BigInt" data-x-href="https://tc39.github.io/proposal-bigint/#sec-ecmascript-language-types-bigint-type">BigInt</dfn> type.</li>
+     <li>The <dfn data-x="js-BigIntData" data-x-href="https://tc39.github.io/proposal-bigint/#sec-toobject">[[BigIntData]]</dfn> internal slot</li>
+    </ul>
+
    </dd>
 
 
@@ -8127,7 +8136,8 @@ interface <dfn>DOMStringList</dfn> {
    <li><p>Let <var>deep</var> be false.</p></li>
 
    <li><p>If <span data-x="js-Type">Type</span>(<var>value</var>) is Undefined, Null, Boolean,
-   String, or Number, then return { [[Type]]: "primitive", [[Value]]: <var>value</var> }.</p></li>
+   Number, <span data-x="js-BigInt">BigInt</span>, or String, then return { [[Type]]: "primitive",
+   [[Value]]: <var>value</var> }.</p></li>
 
    <li><p>If <span data-x="js-Type">Type</span>(<var>value</var>) is Symbol, then throw a
    <span>"<code>DataCloneError</code>"</span> <code>DOMException</code>.</p></li>
@@ -8141,6 +8151,10 @@ interface <dfn>DOMStringList</dfn> {
    <var>serialized</var> to { [[Type]]: "Number", [[NumberData]]: <var>value</var>.[[NumberData]]
    }.</p></li>
 
+   <li><p>Otherwise, if <var>value</var> has a <span data-x="js-BigIntData">[[BigIntData]]</span>
+   internal slot, then set <var>serialized</var> to { [[Type]]: "BigInt", [[BigIntData]]:
+   <var>value</var>.<span data-x="js-BigIntData">[[BigIntData]]</span> }.</p></li>
+
    <li><p>Otherwise, if <var>value</var> has a [[StringData]] internal slot, then set
    <var>serialized</var> to { [[Type]]: "String", [[StringData]]: <var>value</var>.[[StringData]]
    }.</p></li>
@@ -8546,6 +8560,10 @@ o.myself = o;</pre>
    <var>targetRealm</var> whose [[NumberData]] internal slot value is
    <var>serialized</var>.[[NumberData]].</p></li>
 
+   <li><p>Otherwise, if <var>serialized</var>.[[Type]] is "BigInt", then set <var>value</var> to a new BigInt object in
+   <var>targetRealm</var> whose <span data-x="js-BigIntData">[[BigIntData]]</span> internal slot value is
+   <var>serialized</var>.[[BigIntData]].</p></li>
+
    <li><p>Otherwise, if <var>serialized</var>.[[Type]] is "String", then set <var>value</var> to a new String object in
    <var>targetRealm</var> whose [[StringData]] internal slot value is
    <var>serialized</var>.[[StringData]].</p></li>
@@ -120209,6 +120227,9 @@ INSERT INTERFACES HERE
    <dt id="refsJPEG">[JPEG]</dt>
    <dd><cite><a href="https://www.w3.org/Graphics/JPEG/jfif3.pdf">JPEG File Interchange Format</a></cite>, E. Hamilton.</dd>
 
+   <dt id="refsJSBIGINT">[JSBIGINT]</dt>
+   <dd><cite><a href="https://tc39.github.io/proposal-bigint/">BigInt</a></cite>. Ecma International.</dd>
+
    <dt id="refsJSIMPORT">[JSIMPORT]</dt>
    <dd><cite><a href="https://tc39.github.io/proposal-dynamic-import/">import()</a></cite>. Ecma International.</dd>