Re-land "Search for main in the exported names of the main library, not in the library itself."

tests/language/export_double_same_main_test.dart

Index: tests/language/export_double_same_main_test.dart
diff --git a/pkg/shadow_dom/lib/src/platform/platform-init.js b/tests/language/export_double_same_main_test.dart
similarity index 73%
copy from pkg/shadow_dom/lib/src/platform/platform-init.js
copy to tests/language/export_double_same_main_test.dart
index 4e83afc8c5fafb541d0e44d97fbd3621ddba5040..68ad6003113b5c22e47951df8cda586fddf59b56 100644
--- a/pkg/shadow_dom/lib/src/platform/platform-init.js
+++ b/tests/language/export_double_same_main_test.dart
@@ -2,4 +2,5 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-var Platform = {};
+export 'top_level_entry_test.dart';
+export 'export_main_test.dart' show main;