[flutter_tools] Fix breakage when test main is tagged with @doNotStore (#79501)

packages/flutter_tools/lib/src/test/flutter_platform.dart

@@ -182,8 +182,11 @@ Future<void> _testMain() async {
   IntegrationTestWidgetsFlutterBinding.ensureInitialized();
 ''');
   }
+  // Don't propagate the return value of `test.main` here. If the `main`
+  // function on users` test is annotated with `@doNotStore`, it will cause an
+  // analyzer error otherwise.
   buffer.write('''
-  return test.main();
+  await Future(test.main);
 }
 
 /// Capture any top-level errors (mostly lazy syntax errors, since other are