Fix -sMODULARIZE + -sLEGACY_VM_SUPPORT

When we run babel in `-sMODULARIZE` mode the input to babel contains
`await` calls.  This code appears to be top-level code at the point
at which we run babel since we have yet to perform the modularization
process which wraps the output in a factory function.

Fixes: emscripten-core#23687

test/test_other.py

@@ -7056,6 +7056,9 @@ 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/building.py

@@ -511,7 +511,7 @@ def version_split(v):
 @ToolchainProfiler.profile()
 def transpile(filename):
   config = {
-    'sourceType': 'script',
+    'sourceType': 'module',
     'targets': {}
   }
   if settings.MIN_CHROME_VERSION != UNSUPPORTED: