Fix typo when generating a not boolified input.

frog/tests/leg/boolify_test.dart

+// 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.
+
+#library("boolified_operator_test.dart");
+#import("compiler_helper.dart");
+
+final String TEST = @"""
+foo() {
+  var a = foo();
+  if (!a) return 1;
+  return 2;
+}
+""";
+
+main() {
+  String generated = compile(TEST, 'foo');
+  Expect.isTrue(generated.contains('foo() !== true)'));
+}