Don't double-escape in strings in the VM Service, and don't use \u0000 to determine the string lengt

runtime/vm/service_test.cc

 // 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.
 
 #include "platform/globals.h"
 
 #include "include/dart_debugger_api.h"
 #include "vm/dart_api_impl.h"
 #include "vm/dart_entry.h"
 #include "vm/debugger.h"
@@ skipping 602 matching lines @@
   handler.filterMsg("id");
   EXPECT_STREQ(
       "{\"type\":\"Array\","
       "\"class\":{\"type\":\"@Class\",\"name\":\"_List\",},"
       "\"fields\":[],"
       "\"length\":1,"
       "\"elements\":[{"
           "\"index\":0,"
           "\"value\":{\"type\":\"@String\","
           "\"class\":{\"type\":\"@Class\",\"name\":\"_OneByteString\",},"
-          "\"valueAsString\":\"\\\"value\\\"\"}}]}",
+          "\"valueAsString\":\"value\","
+          "\"valueAsStringIsTruncated\":false}}]}",
       handler.msg());
 
   // object id ring / invalid => expired
   service_msg = Eval(lib, "[0, port, ['objects', '99999999'], [], []]");
   Service::HandleIsolateMessage(isolate, service_msg);
   handler.HandleNextMessage();
   handler.filterMsg("vmName");
   EXPECT_STREQ(
       "{\"type\":\"Sentinel\",\"id\":\"objects\\/expired\","
       "\"valueAsString\":\"<expired>\"}",
@@ skipping 1391 matching lines @@
   service_msg = Eval(lib, "[0, port, ['profile'], ['tags'], ['hidden']]");
   Service::HandleIsolateMessage(isolate, service_msg);
   handler.HandleNextMessage();
   // Expect error.
   EXPECT_SUBSTRING("\"type\":\"Error\"", handler.msg());
 }
 
 #endif  // !defined(TARGET_ARCH_ARM64)
 
 }  // namespace dart