[flutter_tools] Reland: fix multiple dart defines (#54973)

dev/integration_tests/web/lib/web_define_loading.dart

@@ -0,0 +1,24 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:html' as html;
+
+Future<void> main() async {
+  final StringBuffer output = StringBuffer();
+  const String combined = String.fromEnvironment('test.valueA') +
+    String.fromEnvironment('test.valueB');
+  if (combined == 'ExampleValue') {
+    output.write('--- TEST SUCCEEDED ---');
+    print('--- TEST SUCCEEDED ---');
+  } else {
+    output.write('--- TEST FAILED ---');
+    print('--- TEST FAILED ---');
+  }
+
+  html.HttpRequest.request(
+    '/test-result',
+    method: 'POST',
+    sendData: '$output',
+  );
+}