fixup: assert baseURL is a string for network modules

Author: bmeck

lib/internal/modules/esm/loader.js

@@ -23,6 +23,7 @@ const {
 const { MessageChannel } = require('internal/worker/io');
 
 const {
+  ERR_INTERNAL_ASSERTION,
   ERR_INVALID_ARG_TYPE,
   ERR_INVALID_ARG_VALUE,
   ERR_INVALID_RETURN_PROPERTY_VALUE,
@@ -260,6 +261,10 @@ class ESMLoader {
       // fetchModule's cache, in which case, fetchModule returns
       // immediately and synchronously
       url = fetchModule(new URL(url), { parentURL: url }).resolvedHREF;
+      // This should only occur if the module hasn't been fetched yet
+      if (typeof url !== 'string') {
+        throw new ERR_INTERNAL_ASSERTION(`Base url for module ${url} not loaded.`);
+      }
     }
     return url;
   }