Add a new node in the inferrer to materialize literal maps.

tests/language/list_tracer_in_map_test.dart

Index: tests/language/list_tracer_in_map_test.dart
===================================================================
--- tests/language/list_tracer_in_map_test.dart	(revision 0)
+++ tests/language/list_tracer_in_map_test.dart	(revision 0)
@@ -0,0 +1,14 @@
+// Copyright (c) 2013, 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 dart2js, whose type inferrer used to not see
+// literal maps as places where a list could escape.
+
+var b = [42];
+var a = {'foo': b};
+
+main() {
+  a['foo'].clear();
+  if (b.length != 0) throw 'Test failed';
+}