Call 'use' instead of 'visit' on the input when generating code for an unchecked HTypeConversion.

dart/tests/language/type_guard_conversion_test.dart

Index: dart/tests/language/type_guard_conversion_test.dart
===================================================================
--- dart/tests/language/type_guard_conversion_test.dart	(revision 0)
+++ dart/tests/language/type_guard_conversion_test.dart	(revision 0)
@@ -0,0 +1,18 @@
+// Copyright (c) 2012, 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() => 'bar';
+
+main() {
+  var a = foo();
+  var b = 'c';
+  do {
+    b = a[2];
+  } while (b != 'r');
+
+  if (a is Hashable) {
+    a = a.concat(a);
+  }
+  Expect.equals('barbar', a);
+}