Fix a couple codegen crashers

test/codegen/try_catch.dart

 // Copyright (c) 2015, 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.
 
 foo() {
   try {
     throw "hi there";
   } on String catch (e, t) {
   } catch (e, t) {
     rethrow;
@@ skipping 20 matching lines @@
 
 qux() {
   try {
     throw "on only";
   } on String catch (e, t) {
     // unreachable
     rethrow;
   }
 }
 
+wub() {
+  try {
+    throw "on without type";

[Jennifer Messerly, 2015/04/10 20:32:33]

"on without exception parameter" ?

[vsm, 2015/04/10 20:41:35]

Done.

+  } on String {
+  }
+}
+
 main() {
   foo();
   bar();
   baz();
   qux();
+  wub();
 }