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

pkg/front_end/testcases/inference/infer_throw_downwards.dart.strong.expect

Index: pkg/front_end/testcases/inference/infer_throw_downwards.dart.strong.expect
diff --git a/pkg/front_end/testcases/inference/infer_throw_downwards.dart.strong.expect b/pkg/front_end/testcases/inference/infer_throw_downwards.dart.strong.expect
new file mode 100644
index 0000000000000000000000000000000000000000..7e9227c531476fc4e42cab6ab27255feb6169a08
--- /dev/null
+++ b/pkg/front_end/testcases/inference/infer_throw_downwards.dart.strong.expect
@@ -0,0 +1,11 @@
+library test;
+import self as self;
+import "dart:core" as core;
+
+static field dynamic x = throw self::f<dynamic>();
+static method f<T extends core::Object>() → self::f::T
+  return null;
+static method g() → void {
+  dynamic x = throw self::f<dynamic>();
+}
+static method main() → dynamic {}