Add missing verified write on x64; extend test coverage

tests/lib/convert/streamed_conversion_json_utf8_decode_test.dart

 // Copyright (c) 2013, 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 --old_gen_growth_rate=1
+// VMOptions=
 
 import "package:expect/expect.dart";
 import 'dart:async';
 import 'dart:convert';
 import 'json_unicode_tests.dart';
 import "package:async_helper/async_helper.dart";
 
 final JSON_UTF8 = JSON.fuse(UTF8);
 
 void expectJsonEquals(o1, o2, [path = "result"]) {
@@ skipping 57 matching lines @@
     var bytes = test[0];
     var o = test[1];
     checkIsJsonEqual(o, decode(bytes));
     checkIsJsonEqual(o, decodeChunked(bytes, 1));
     checkIsJsonEqual(o, decodeChunked(bytes, 2));
     checkIsJsonEqual(o, decodeChunked(bytes, 3));
     checkIsJsonEqual(o, decodeChunked(bytes, 4));
     checkIsJsonEqual(o, decodeChunked(bytes, 5));
   }
 }