Add Closure compiler support for ESM dynamic imports

Closure compiler runs before the MODULARIZE async wrapper is applied,
so it sees `await import('node:xyz')` outside an async function and
fails to parse it.

Work around this by:
1. Before Closure: Replace `await import('node:xyz')` with placeholder
   variables like `__EMSCRIPTEN_PRIVATE_AWAIT_IMPORT_xyz__`
2. Generate externs for the placeholders so Closure doesn't error on
   undeclared variables
3. After Closure: Restore placeholders to `(await import('node:xyz'))`
   with parentheses to handle cases where Closure inlines the variable
   into expressions like `placeholder.method()`

This follows the same pattern as the existing
`__EMSCRIPTEN_PRIVATE_MODULE_EXPORT_NAME_SUBSTITUTION__` mechanism.

Author: Bartek Wrona

tools/link.py

@@ -2322,7 +2322,43 @@ def phase_binaryen(target, options, wasm_target):
 
     if options.use_closure_compiler:
       with ToolchainProfiler.profile_block('closure_compile'):
-        final_js = building.closure_compiler(final_js, extra_closure_args=settings.CLOSURE_ARGS)
+        # In EXPORT_ES6 mode, we use `await import('node:xyz')` for dynamic imports.
+        # Closure compiler runs before the MODULARIZE async wrapper is applied, so it
+        # sees `await` outside an async function and fails to parse it.
+        # Work around this by replacing dynamic imports with placeholders before Closure,
+        # then restoring them after.
+        closure_args = list(settings.CLOSURE_ARGS) if settings.CLOSURE_ARGS else []
+        if settings.EXPORT_ES6:
+          src = utils.read_file(final_js)
+          # Find all node modules being dynamically imported (with optional bundler hints)
+          node_modules = set(re.findall(r"await\s+import\(\s*(?:/\*.*?\*/\s*)*['\"]node:(\w+)['\"]\s*\)", src))
+          if node_modules:
+            # Replace: await import(/* ... */ 'node:fs') -> __EMSCRIPTEN_PRIVATE_AWAIT_IMPORT_fs__
+            src = re.sub(r"await\s+import\(\s*(?:/\*.*?\*/\s*)*['\"]node:(\w+)['\"]\s*\)",
+                         r'__EMSCRIPTEN_PRIVATE_AWAIT_IMPORT_\1__', src)
+            utils.write_file(final_js, src)
+            save_intermediate('closure_pre')
+            # Generate externs for the placeholder variables so Closure doesn't complain
+            externs_content = '\n'.join(
+              f'/** @type {{?}} */ var __EMSCRIPTEN_PRIVATE_AWAIT_IMPORT_{mod}__;'
+              for mod in node_modules
+            )
+            externs_file = shared.get_temp_files().get('.js', prefix='node_import_externs_')
+            externs_file.write(externs_content.encode())
+            externs_file.close()
+            closure_args += ['--externs', externs_file.name]
+
+        final_js = building.closure_compiler(final_js, extra_closure_args=closure_args)
+
+        if settings.EXPORT_ES6:
+          src = utils.read_file(final_js)
+          # Restore: __EMSCRIPTEN_PRIVATE_AWAIT_IMPORT_fs__ -> (await import(/* hints */ 'node:fs'))
+          # Parentheses are needed because Closure may inline the placeholder into
+          # expressions like `placeholder.method()` which needs `(await import(...)).method()`
+          src = re.sub(r'__EMSCRIPTEN_PRIVATE_AWAIT_IMPORT_(\w+)__',
+                       r"(await import(/* webpackIgnore: true */ /* @vite-ignore */ 'node:\1'))", src)
+          utils.write_file(final_js, src)
+
       save_intermediate('closure')
 
     if settings.TRANSPILE: