Add/fix/cleanup type inference for throw and rethrow.

pkg/front_end/testcases/inference/infer_rethrow.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.
+
+/*@testedFeatures=inference*/
+library test;
+
+// "rethrow" is a statement, so there aren't any types to infer.  This test just
+// exercises the code path to make sure it doesn't crash.
+
+test(f(), g()) {
+  try {
+    f();
+  } catch (_) {
+    g();
+    rethrow;
+  }
+}
+
+main() {}