Introduce initial plumbing for type promotion in fasta.

pkg/front_end/testcases/inference/type_promotion_stopped_by_assignment_in_scope.dart

Index: pkg/front_end/testcases/inference/type_promotion_stopped_by_assignment_in_scope.dart
diff --git a/pkg/front_end/testcases/inference/type_promotion_stopped_by_assignment_in_scope.dart b/pkg/front_end/testcases/inference/type_promotion_stopped_by_assignment_in_scope.dart
new file mode 100644
index 0000000000000000000000000000000000000000..3f273a3a68fafd51700ff0a57a796cdc1ba16f62
--- /dev/null
+++ b/pkg/front_end/testcases/inference/type_promotion_stopped_by_assignment_in_scope.dart
@@ -0,0 +1,12 @@
+// 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;
+  }
+}