Respond to Ivan's code review comments from previous TBR cl.

runtime/vm/object_test.cc

 // 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.
 
 #include "vm/assembler.h"
 #include "vm/bigint_operations.h"
 #include "vm/class_finalizer.h"
 #include "vm/dart_api_impl.h"
 #include "vm/dart_entry.h"
 #include "vm/isolate.h"
@@ skipping 1526 matching lines @@
   const String& str =
       String::Handle(
           OneByteString::New(characters, len, Heap::kNew));
   EXPECT(str.IsOneByteString());
   EXPECT_EQ(str.Length(), len);
   EXPECT(str.Equals("a\n\f\b\t\v\r\\$z"));
   const String& escaped_str =
       String::Handle(String::EscapeSpecialCharacters(str));
   EXPECT(escaped_str.Equals("a\\n\\f\\b\\t\\v\\r\\\\\\$z"));
 
-  const String& empty_str = String::Handle(
-      OneByteString::New(static_cast<intptr_t>(0), Heap::kNew));
   const String& escaped_empty_str =
-      String::Handle(String::EscapeSpecialCharacters(empty_str));
-  EXPECT_EQ(empty_str.Length(), 0);
+      String::Handle(String::EscapeSpecialCharacters(Symbols::Empty()));
   EXPECT_EQ(escaped_empty_str.Length(), 0);
 }
 
 
 TEST_CASE(EscapeSpecialCharactersExternalOneByteString) {
   uint8_t characters[] =
       { 'a', '\n', '\f', '\b', '\t', '\v', '\r', '\\', '$', 'z' };
   intptr_t len = ARRAY_SIZE(characters);
 
   const String& str =
@@ skipping 2011 matching lines @@
   cls = Object::dynamic_class();
   array = cls.fields();
   EXPECT(!array.IsNull());
   EXPECT(array.IsArray());
   array = cls.functions();
   EXPECT(!array.IsNull());
   EXPECT(array.IsArray());
 }
 
 }  // namespace dart