Provide standalone ParseScript wrapper

For use by whatwg/fetch#1442.

source

@@ -94269,14 +94269,8 @@ document.querySelector("button").addEventListener("click", bound);
    <li><p>Set <var>script</var>'s <span data-x="concept-script-parse-error">parse error</span> and
    <span data-x="concept-script-error-to-rethrow">error to rethrow</span> to null.</p></li>
 
-   <li>
-    <p>Let <var>result</var> be <span data-x="js-ParseScript">ParseScript</span>(<var>source</var>,
-    <var>settings</var>'s <span data-x="environment settings object's Realm">Realm</span>,
-    <var>script</var>).</p>
-
-    <p class="note">Passing <var>script</var> as the last parameter here ensures
-    <var>result</var>.[[HostDefined]] will be <var>script</var>.</p>
-   </li>
+   <li><p>Let <var>result</var> be the result of <span>parsing a classic script</span> given
+   <var>source</var> and <var>script</var>.</p></li>
 
    <li>
     <p>If <var>result</var> is a <span>list</span> of errors, then:
@@ -94296,6 +94290,26 @@ document.querySelector("button").addEventListener("click", bound);
    <li><p>Return <var>script</var>.</p></li>
   </ol>
 
+  <p>To <dfn export data-x="parsing a classic script">parse a classic script</dfn>, given a
+  <span>string</span> <var>source</var> and an optional <span>classic script</span>
+  <var>script</var>:</p>
+
+  <ol>
+   <li><p>Let <var>realm</var> be undefined.</p></li>
+
+   <li><p>Let <var>hostDefined</var> be empty.</p></li>
+
+   <li><p>If <var>script</var> is given, then set <var>realm</var> to <var>script</var>'s
+   <span>settings object</span>'s <span data-x="environment settings object's Realm">Realm</span>
+   and <var>hostDefined</var> to <var>script</var>.</p></li>
+
+   <li><p>Return <span data-x="js-ParseScript">ParseScript</span>(<var>source</var>,
+   <var>realm</var>, <var>hostDefined</var>).</p></li>
+  </ol>
+
+  <p class="note"><cite>Fetch</cite> uses <span>parsing a classic script</span> as part of
+  opaque-response blocking. <ref spec=FETCH></p>
+
   <p>To <dfn data-x="creating a JavaScript module script">create a JavaScript module script</dfn>,
   given a <span>string</span> <var>source</var>, an <span>environment settings
   object</span> <var>settings</var>, a <span>URL</span> <var>baseURL</var>, and some <span>script