Support fetch priority with import maps (#2477)

src/features/import-maps.js

@@ -50,7 +50,7 @@ function processScripts () {
     else if (script.type === 'systemjs-importmap') {
       script.sp = true;
       // The passThrough property is for letting the module types fetch implementation know that this is not a SystemJS module.
-      var fetchPromise = script.src ? (System.fetch || fetch)(script.src, { integrity: script.integrity, passThrough: true }).then(function (res) {
+      var fetchPromise = script.src ? (System.fetch || fetch)(script.src, { integrity: script.integrity, priority: script.fetchPriority, passThrough: true }).then(function (res) {
         if (!res.ok)
           throw Error(process.env.SYSTEM_PRODUCTION ? res.status : 'Invalid status code: ' + res.status);
         return res.text();