Make import statements set the importing script as the referrer

Fixes #1150. Previously, they would set the top-level document (that
requested the root of the module script tree) as the referrer. This
makes more sense, being for example consistent with CSS @import.

source

@@ -85978,23 +85978,26 @@ interface <dfn>NavigatorOnLine</dfn> {
 
   <p>To <dfn>fetch a module script tree</dfn> given a <var>url</var>, a <var>credentials mode</var>,
   a <var>cryptographic nonce</var>, a <var>parser state</var>, a <var>destination</var>, a
-  <var>fetch client settings object</var>, and an optional <var>ancestor list</var> and <var>module
-  map settings object</var>, run these steps. The algorithm will asynchronously complete with either
-  null (on failure) or a <span>module script</span> (on success).</p>
+  <var>fetch client settings object</var>, and optional <var>module map settings object</var>,
+  <var>ancestor list</var>, and <var>referrer</var>, run these steps. The algorithm will
+  asynchronously complete with either null (on failure) or a <span>module script</span> (on
+  success).</p>
 
   <ol>
-   <li><p>If <var>ancestor list</var> is not given, let it be an empty list.</p></li>
-
    <li><p>If <var>module map settings object</var> is not given, let it be <var>fetch client
    settings object</var>.</p></li>
 
+   <li><p>If <var>ancestor list</var> is not given, let it be an empty list.</p></li>
+
+   <li><p>If <var>referrer</var> is not given, let it be "<code data-x="">client</code>".</p>
+
    <li><p><span>Fetch a single module script</span> given <var>url</var>, <var>credentials
    mode</var>, <var>cryptographic nonce</var>, <var>parser state</var>, <var>destination</var>,
-   <var>fetch client settings object</var>, and <var>module map settings object</var>. If the caller
-   of this algorithm specified custom <span data-x="fetching-scripts-set-up-request">set up the
-   request</span> or <span data-x="fetching-scripts-process-response">process the response</span>
-   steps, pass those along while <span data-x="fetch a single module script">fetching a single
-   module script</span>.</p>
+   <var>fetch client settings object</var>, <var>module map settings object</var>, and
+   <var>referrer</var>. If the caller of this algorithm specified custom <span
+   data-x="fetching-scripts-set-up-request">set up the request</span> or <span
+   data-x="fetching-scripts-process-response">process the response</span> steps, pass those along
+   while <span data-x="fetch a single module script">fetching a single module script</span>.</p>
 
    <li><p>Return from this algorithm, and run the following steps when <span data-x="fetch a single
    module script">fetching a single module script</span> asynchronously completes with
@@ -86058,8 +86061,9 @@ interface <dfn>NavigatorOnLine</dfn> {
     data-x="concept-module-script-credentials-mode">credentials mode</span>, <var>module
     script</var>'s <span data-x="concept-module-script-nonce">cryptographic nonce</span>,
     <var>module script</var>'s <span data-x="concept-module-script-parser">parser state</span>,
-    <var>destination</var>, <var>module script</var>'s <span>settings object</span>, and
-    <var>ancestor list</var>.</p>
+    <var>destination</var>, <var>module script</var>'s <span>settings object</span>, <var>module
+    script</var>'s <span>settings object</span>, <var>ancestor list</var>, and <var>module
+    script</var>'s <span data-x="concept-module-script-base-url">base URL</span>.</p>
 
     <p class="note">It is intentional that no custom <span
     data-x="fetching-scripts-set-up-request">set up the request</span> or <span
@@ -86078,9 +86082,9 @@ interface <dfn>NavigatorOnLine</dfn> {
 
   <p>To <dfn>fetch a single module script</dfn>, given a <var>url</var>, a <var>credentials
   mode</var>, a <var>cryptographic nonce</var>, a <var>parser state</var>, a <var>destination</var>,
-  a <var>fetch client settings object</var>, and a <var>module map settings object</var>, run these
-  steps. The algorithm will asynchronously complete with either null (on failure) or a <span>module
-  script</span> (on success).</p>
+  a <var>fetch client settings object</var>, a <var>module map settings object</var>, and a
+  <var>referrer</var>, run these steps. The algorithm will asynchronously complete with either null
+  (on failure) or a <span>module script</span> (on success).</p>
 
   <ol>
    <li><p>Let <var>module map</var> be <var>module map settings object</var>'s <span>module
@@ -86104,8 +86108,10 @@ interface <dfn>NavigatorOnLine</dfn> {
    data-x="concept-request-credentials-mode">credentials mode</span> is <var>credentials
    mode</var>, <span data-x="concept-request-nonce-metadata">cryptographic nonce metadata</span> is
    <var>cryptographic nonce</var>, <span data-x="concept-request-parser-metadata">parser
-   metadata</span> is <var>parser state</var>and <span data-x="concept-request-client">client</span>
-   is <var>fetch client settings object</var>.</p></li>
+   metadata</span> is <var>parser state</var>, <span
+   data-x="concept-request-referrer">referrer</span> is <var>referrer</var>, and <span
+   data-x="concept-request-client">client</span> is <var>fetch client settings
+   object</var>.</p></li>
 
    <li><p>If the caller specified custom steps to <span data-x="fetching-scripts-set-up-request">set
    up the request</span>, perform them on <var>request</var>.</p></li>
@@ -86136,7 +86142,7 @@ interface <dfn>NavigatorOnLine</dfn> {
      <li>
       <p>The result of <span data-x="extract a MIME type">extracting a MIME type</span> from
       <var>response</var>'s <span data-x="concept-response-header-list">header list</span>
-      (ignoring parameters) is not a <span>JavaScript MIME type</span>.</p>
+      (ignoring parameters) is not a <span>JavaScript MIME type</span></p>
 
       <p class="note">For historical reasons, <span data-x="fetch a classic script">fetching a
       classic script</span> does not include MIME type checking. In contrast, module scripts will
@@ -86145,7 +86151,7 @@ interface <dfn>NavigatorOnLine</dfn> {
 
      <li><p>The caller specified custom steps to <span
      data-x="fetching-scripts-process-response">process the response</span>, which when performed
-     on <var>response</var> return false.</p></li>
+     on <var>response</var> return false</p></li>
     </ul>
    </li>