Enable createWasm to use async/await with closure

I really which we had a better solution here, but this makes the use
of async/await independent of closure usage.

This is a workaround for emscripten-core#23158, but it also happens to fix emscripten-core#23687
which stems from the same issue.

Fixes: emscripten-core#23687

src/closure-externs/closure-externs.js

@@ -14,6 +14,7 @@
 // Special placeholder for `import.meta` and `await import`.
 var EMSCRIPTEN$IMPORT$META;
 var EMSCRIPTEN$AWAIT$IMPORT;
+var EMSCRIPTEN$AWAIT;
 
 // Don't minify createRequire
 var createRequire;

test/other/codesize/test_codesize_minimal_esm.gzsize

@@ -1 +1 @@
-1436
+1431

test/other/codesize/test_codesize_minimal_esm.jssize

@@ -1 +1 @@
-2995
+2980

test/test_other.py

@@ -7098,6 +7098,9 @@ class Descriptor {
     self.run_process([EMXX, 'src.cpp', '-O2', '-sEXPORT_ALL'])
     self.assertExists('a.out.js')
 
+  def test_modularize_legacy(self):
+    self.do_runf('hello_world.c', emcc_args=['-sMODULARIZE', '-sLEGACY_VM_SUPPORT'])
+
   def test_emmake_emconfigure(self):
     def check(what, args, fail=True, expect=''):
       args = [what] + args

tools/emscripten.py

@@ -877,13 +877,7 @@ def create_sending(metadata, library_symbols):
 
 
 def can_use_await():
-  # In MODULARIZE mode we can use `await` since the factory function itself
-  # is marked as `async` and the generated code all lives inside that factory
-  # function.
-  # However, because closure does not see this (it runs only on the inner code),
-  # it sees this as a top-level-await, which it does not yet support.
-  # FIXME(https://github.com/emscripten-core/emscripten/issues/23158)
-  return settings.MODULARIZE and not settings.USE_CLOSURE_COMPILER
+  return settings.MODULARIZE
 
 
 def make_export_wrappers(function_exports):
@@ -993,7 +987,10 @@ def create_module(receiving, metadata, global_exports, library_symbols):
     if settings.WASM_ASYNC_COMPILATION:
       if can_use_await():
         # In modularize mode the generated code is within a factory function.
-        module.append("var wasmExports = await createWasm();\n")
+        # This magic string gets replaced by `await createWasm`.  It needed to allow
+        # closure and acorn to process the module without seeing this as a top-level
+        # await.
+        module.append("var wasmExports = EMSCRIPTEN$AWAIT(createWasm());\n")
       else:
         module.append("var wasmExports;\ncreateWasm();\n")
     else:

tools/link.py

@@ -2065,13 +2065,15 @@ def phase_source_transforms(options):
 # both main code and libraries.
 # See also: `preprocess` in parseTools.js.
 def fix_es6_import_statements(js_file):
-  if not settings.EXPORT_ES6 or not settings.USE_ES6_IMPORT_META:
+  if not settings.MODULARIZE:
     return
 
   src = read_file(js_file)
   write_file(js_file, src
              .replace('EMSCRIPTEN$IMPORT$META', 'import.meta')
-             .replace('EMSCRIPTEN$AWAIT$IMPORT', 'await import'))
+             .replace('EMSCRIPTEN$AWAIT$IMPORT', 'await import')
+             .replace('EMSCRIPTEN$AWAIT(createWasm())', 'await createWasm()')
+             .replace('EMSCRIPTEN$AWAIT(', 'await ('))
   save_intermediate('es6-module')