Add missing verified write on x64; extend test coverage

tests/standalone/verified_mem_test.dart

 // Copyright (c) 2014, 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.
 //
 // Test write barrier verification mode.
-// VMOptions=--verified_mem --verify_before_gc --verify_after_gc
+// VMOptions=--verified_mem --verify_before_gc --verify_after_gc --old_gen_growth_rate=1
 
 var a = [];
 
 void main() {
   for (int i = 0; i < 12; ++i) {
     a.add(new List(12345));
   }
   for (int i = 0; i < 1234; ++i) {
     a[0] = new List(100000);
   }
 }