Introduce initial plumbing for type promotion in fasta.

pkg/front_end/testcases/inference/type_promotion_stopped_by_mutation_in_a_closure.dart

Index: pkg/front_end/testcases/inference/type_promotion_stopped_by_mutation_in_a_closure.dart
diff --git a/pkg/front_end/testcases/inference/type_promotion_stopped_by_mutation_in_a_closure.dart b/pkg/front_end/testcases/inference/type_promotion_stopped_by_mutation_in_a_closure.dart
new file mode 100644
index 0000000000000000000000000000000000000000..da7fc0284ec328e1ed6c016d7b85d25bf44ca974
--- /dev/null
+++ b/pkg/front_end/testcases/inference/type_promotion_stopped_by_mutation_in_a_closure.dart
@@ -0,0 +1,14 @@
+// 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.md file.
+
+/*@testedFeatures=inference*/
+main() {
+  num n = null;
+  if (/*@promotedType=none*/ n is int) {
+    var /*@type=dart.core::num*/ i = /*@promotedType=none*/ n;
+  }
+  () {
+    n = null;
+  };
+}