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

pkg/front_end/testcases/inference/infer_throw_downwards.dart

Index: pkg/front_end/testcases/inference/infer_throw_downwards.dart
diff --git a/pkg/front_end/testcases/inference/infer_throw_downwards.dart b/pkg/front_end/testcases/inference/infer_throw_downwards.dart
new file mode 100644
index 0000000000000000000000000000000000000000..c733531e4f78e74afb38605c089dfe8328c1b43c
--- /dev/null
+++ b/pkg/front_end/testcases/inference/infer_throw_downwards.dart
@@ -0,0 +1,16 @@
+// 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;
+
+T f<T>() => null;
+
+var /*@topType=dynamic*/ x = throw /*@typeArgs=dynamic*/ f();
+
+void g() {
+  var /*@type=dynamic*/ x = throw /*@typeArgs=dynamic*/ f();
+}
+
+main() {}