Fix typo in native_helper and add dartc test.

tests/language/src/GettersSettersTypeTest.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.
+// Getters and setters can have different types, but it is a warning if the
+// two types are not assignable.
+
+int bar = 499;
+
+int  /// 01: static type warning
+get foo() => bar;
+
+void set foo(
+             String   /// 01: continued
+             str) { bar = str.length; }
+
+main() {
+  int x = foo;
+  Expect.equals(499, x);
+  foo = "1234";
+  int y = foo;
+  Expect.equals(4, y);
+}