Type tracing bug fix for issue 28919.

tests/compiler/dart2js_extra/28919_test.dart

+// 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.
+
+// Regression test for http://dartbug.com/28919.
+//
+// This would crash at runtime because the type of closure parameter 'x' was
+// empty, leading to bad codegen. The empty type was due to a tracing bug that
+// failed to escape the closure when stored into the list which was not traced
+// as a container.
+
+import 'package:expect/expect.dart';
+
+int foo([List _methods = const []]) {
+  final methods = new List.from(_methods);
+  for (int i = 0; i < 3; i++) {
+    methods.add((int x) => x + i);
+  }
+  return methods[0](499);
+}
+
+main() {
+  Expect.equals(499, foo());
+}