Minor cleanups to use null_array(), null_object() and null_string() handles.

runtime/vm/object.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/object.h"
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "vm/assembler.h"
 #include "vm/cpu.h"
@@ skipping 5345 matching lines @@
  public:
   static const intptr_t kInitialSize = 16 * KB;
   CompressedTokenStreamData() :
       buffer_(NULL),
       stream_(&buffer_, Reallocate, kInitialSize),
       token_objects_(GrowableObjectArray::Handle(
           GrowableObjectArray::New(kInitialTokenCount, Heap::kOld))),
       token_obj_(Object::Handle()),
       literal_token_(LiteralToken::Handle()),
       literal_str_(String::Handle()) {
-    const String& empty_literal = String::Handle();
-    token_objects_.Add(empty_literal);
+    token_objects_.Add(Object::null_string());
   }
   ~CompressedTokenStreamData() {
   }
 
   // Add an IDENT token into the stream and the token objects array.
   void AddIdentToken(String* ident) {
     if (ident != NULL) {
       // If the IDENT token is already in the tokens object array use the
       // same index instead of duplicating it.
       intptr_t index = FindIdentIndex(ident);
@@ skipping 8251 matching lines @@
                                     space);
   return reinterpret_cast<RawWeakProperty*>(raw);
 }
 
 
 const char* WeakProperty::ToCString() const {
   return "_WeakProperty";
 }
 
 }  // namespace dart