Acorn optimizer: Allow top-level await expressions (#21117)

`-pthread` + `-sEXPORT_ES6` emits a top-level await expression
in the main worker file after commit 49ab23a. Ensure the Acorn
optimizer is aware of this.

test/test_other.py

@@ -407,7 +407,8 @@ def test_export_es6_allows_export_in_post_js(self):
   @parameterized({
     '': ([],),
     # load a worker before startup to check ES6 modules there as well
-    'pthreads': (['-pthread', '-sPTHREAD_POOL_SIZE=1'],),
+    # pass -O2 to ensure the worker JS file is minified with Acorn
+    'pthreads': (['-O2', '-pthread', '-sPTHREAD_POOL_SIZE=1'],),
   })
   def test_export_es6(self, args, package_json):
     self.run_process([EMCC, test_file('hello_world.c'), '-sEXPORT_ES6',

tools/acorn-optimizer.mjs

@@ -2029,6 +2029,7 @@ try {
     preserveParens: closureFriendly,
     onComment: closureFriendly ? sourceComments : undefined,
     sourceType: exportES6 ? 'module' : 'script',
+    allowAwaitOutsideFunction: true,
   });
 } catch (err) {
   err.message += (() => {