Track void context.

test/kernel/regression/for_loop.dart

 // Copyright (c) 2016, 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.md file.
 
 main() {
-  try {
-    print(42);
-  } finally {
-    print(87);
+  int c = new DateTime.now().millisecondsSinceEpoch;
+  for (int i = 0; i < 100; i++) {
+    print(i++);
+  }
+  for (int i = 0; i < 100; c < 42 ? throw "fisk" : i++) {
+    print(i++);
   }
 }