Add DDC regression test. (#90692)

packages/flutter/test/dart/browser_environment_test.dart

+// 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.
+
+@TestOn('browser')
+
+import 'package:flutter_test/flutter_test.dart';
+
+// Regression test for: https://github.com/dart-lang/sdk/issues/47207
+// Needs: https://github.com/dart-lang/sdk/commit/6c4593929f067af259113eae5dc1b3b1c04f1035 to pass.
+// Originally here: https://github.com/flutter/engine/pull/28808
+void main() {
+  test('Web library environment define exists', () {
+    expect(const bool.fromEnvironment('dart.library.html'), isTrue);
+    expect(const bool.fromEnvironment('dart.library.someFooLibrary'), isFalse);
+  });
+}