Track async causal stack traces

tests/standalone/dart_developer_env_test.dart

Index: tests/standalone/dart_developer_env_test.dart
diff --git a/tests/standalone/dart_developer_env_test.dart b/tests/standalone/dart_developer_env_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..1d0a276ccdd45f909e3cee0a58c9c26f7804431e
--- /dev/null
+++ b/tests/standalone/dart_developer_env_test.dart
@@ -0,0 +1,10 @@
+// Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
+// 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.
+
+import "package:expect/expect.dart";
+
+main() {
+  Expect.isTrue(
+      const bool.fromEnvironment('dart.developer.causal_async_stacks'));
+}