Do not run FinishDynamicImport steps in parallel

These steps do a variety of things that only make sense back on the main thread.
The intention here was that fetching not block the main thread. However, that is
already the case even without going in parallel, given the "async algorithm"
framework of the script-fetching algorithms. (That framework is admittedly a bit
unclear; see whatwg/infra#181.)

source

@@ -88519,63 +88519,57 @@ import "https://example.com/foo/../module2.mjs";</code></pre>
    <li><p>Let <var>referencing script</var> be
    <var>referencingScriptOrModule</var>.[[HostDefined]].</p></li>
 
+   <li><p>Let <var>url</var> be the result of <span data-x="resolve a module specifier">resolving a
+   module specifier</span> given <var>referencing script</var>'s <span
+   data-x="concept-script-base-url">base URL</span> and <var>specifier</var>.</p></li>
+
    <li>
-    <p>Run the following steps <span>in parallel</span>:</p>
+    <p>If <var>url</var> is failure, then:</p>
 
     <ol>
-     <li><p>Let <var>url</var> be the result of <span data-x="resolve a module specifier">resolving a
-     module specifier</span> given <var>referencing script</var>'s <span
-     data-x="concept-script-base-url">base URL</span> and <var>specifier</var>.</p></li>
+     <li><p>Let <var>completion</var> be Completion { [[Type]]: throw, [[Value]]: a new
+     <code>TypeError</code>, [[Target]]: empty }.</p></li>
 
-     <li>
-      <p>If <var>url</var> is failure, then:</p>
+     <li><p>Perform <span>FinishDynamicImport</span>(<var>referencingScriptOrModule</var>,
+     <var>specifier</var>, <var>promiseCapability</var>, <var>completion</var>).</p></li>
 
-      <ol>
-       <li><p>Let <var>completion</var> be Completion { [[Type]]: throw, [[Value]]: a new
-       <code>TypeError</code>, [[Target]]: empty }.</p></li>
-
-       <li><p>Perform <span>FinishDynamicImport</span>(<var>referencingScriptOrModule</var>,
-       <var>specifier</var>, <var>promiseCapability</var>, <var>completion</var>).</p></li>
-
-       <li><p>Return.</p></li>
-      </ol>
-     </li>
+     <li><p>Return.</p></li>
+    </ol>
+   </li>
 
-     <li><p>Let <var>options</var> be the <span>descendant script fetch options</span> for
-     <var>referencing script</var>'s <span data-x="concept-script-script-fetch-options">fetch
-     options</span>.</p></li>
+   <li><p>Let <var>options</var> be the <span>descendant script fetch options</span> for
+   <var>referencing script</var>'s <span data-x="concept-script-script-fetch-options">fetch
+   options</span>.</p></li>
 
-     <li><p><span>Fetch a module script graph</span> given <var>url</var>, <var>referencing
-     script</var>'s <span>settings object</span>, "<code data-x="">script</code>", and
-     <var>options</var>. Wait until the algorithm asynchronously completes with
-     <var>result</var>.</p></li>
+   <li><p><span>Fetch a module script graph</span> given <var>url</var>, <var>referencing
+   script</var>'s <span>settings object</span>, "<code data-x="">script</code>", and
+   <var>options</var>. Wait until the algorithm asynchronously completes with
+   <var>result</var>.</p></li>
 
-     <li>
-      <p>If <var>result</var> is null, then:</p>
+   <li>
+    <p>If <var>result</var> is null, then:</p>
 
-      <ol>
-       <li><p>Let <var>completion</var> be Completion { [[Type]]: throw, [[Value]]: a new
-       <code>TypeError</code>, [[Target]]: empty }.</p></li>
+    <ol>
+     <li><p>Let <var>completion</var> be Completion { [[Type]]: throw, [[Value]]: a new
+     <code>TypeError</code>, [[Target]]: empty }.</p></li>
 
-       <li><p>Perform <span>FinishDynamicImport</span>(<var>referencingScriptOrModule</var>,
-       <var>specifier</var>, <var>promiseCapability</var>, <var>completion</var>).</p></li>
+     <li><p>Perform <span>FinishDynamicImport</span>(<var>referencingScriptOrModule</var>,
+     <var>specifier</var>, <var>promiseCapability</var>, <var>completion</var>).</p></li>
 
-       <li><p>Return.</p></li>
-      </ol>
-     </li>
+     <li><p>Return.</p></li>
+    </ol>
+   </li>
 
-     <li><p><span data-x="run a module script">Run the module script</span> <var>result</var>, with
-     the rethrow errors boolean set to true.</p></li>
+   <li><p><span data-x="run a module script">Run the module script</span> <var>result</var>, with
+   the rethrow errors boolean set to true.</p></li>
 
-     <li><p>If running the module script throws an exception, then perform
-     <span>FinishDynamicImport</span>(<var>referencingScriptOrModule</var>, <var>specifier</var>,
-     <var>promiseCapability</var>, the thrown exception completion).</p></li>
+   <li><p>If running the module script throws an exception, then perform
+   <span>FinishDynamicImport</span>(<var>referencingScriptOrModule</var>, <var>specifier</var>,
+   <var>promiseCapability</var>, the thrown exception completion).</p></li>
 
-     <li><p>Otherwise, perform
-     <span>FinishDynamicImport</span>(<var>referencingScriptOrModule</var>, <var>specifier</var>,
-     <var>promiseCapability</var>, <span>NormalCompletion</span>(undefined)).</p></li>
-    </ol>
-   </li>
+   <li><p>Otherwise, perform
+   <span>FinishDynamicImport</span>(<var>referencingScriptOrModule</var>, <var>specifier</var>,
+   <var>promiseCapability</var>, <span>NormalCompletion</span>(undefined)).</p></li>
 
    <li><p>Return undefined.</p></li>
   </ol>