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;

[Siggi Cherem (dart-lang), 2017/06/23 20:49:27]

i'm probably missing something obvious, but what is this test covering?, that
there are no types being inferred?

[Paul Berry, 2017/06/23 20:56:49]

Yeah, sorry for the confusion.  "rethrow" is a statement, so this test just
exercises the code path to make sure it doesn't crash.

(Confusingly, kernel treats "rethrow" as an expression, so "rethrow;" is
desugared to an expression statement whose expression is a kernel "rethrow"
expression)

I've added a comment to clarify.

+
+test(f(), g()) {
+  try {
+    f();
+  } catch (_) {
+    g();
+    rethrow;
+  }
+}
+
+main() {}