Don't early-exit teardowns after a signal.

lib/src/backend/group.dart

Index: lib/src/backend/group.dart
diff --git a/lib/src/backend/group.dart b/lib/src/backend/group.dart
index 47e06e3aa1eabb6ca6eeb0336d971242a4e730b8..806c3bb7a5d4aa3c3faac94886c60b7b7f5d1e67 100644
--- a/lib/src/backend/group.dart
+++ b/lib/src/backend/group.dart
@@ -74,14 +74,14 @@ class Group {
   /// If no set-up functions are declared, this returns a [Future] that
   /// completes immediately.
   Future runTearDowns() {
-    // TODO(nweiz): Use async/await here once issue 23497 has been fixed in two
-    // stable versions.
-    if (parent == null) {
-      return Future.forEach(tearDowns.reversed, _errorsDontStopTest);
-    }
-
-    return Future.forEach(tearDowns.reversed, _errorsDontStopTest)
-        .then((_) => parent.runTearDowns());
+    return Invoker.current.unclosable(() {
+      var tearDowns = [];
+      for (var group = this; group != null; group = group.parent) {
+        tearDowns.addAll(group.tearDowns.reversed);
+      }
+
+      return Future.forEach(tearDowns, _errorsDontStopTest);
+    });
   }
 
   /// Runs [body] with special error-handling behavior.