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

runtime/vm/flow_graph_compiler.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 "vm/globals.h"  // Needed here to get TARGET_ARCH_XXX.
 
 #include "vm/flow_graph_compiler.h"
 
 #include "vm/cha.h"
 #include "vm/dart_entry.h"
@@ skipping 449 matching lines @@
   const Array& object_array =
       Array::Handle(Array::MakeArray(builder.object_table()));
   ASSERT(code.object_table() == Array::null());
   code.set_object_table(object_array);
 }
 
 
 void FlowGraphCompiler::FinalizeStackmaps(const Code& code) {
   if (stackmap_table_builder_ == NULL) {
     // The unoptimizing compiler has no stack maps.
-    code.set_stackmaps(Array::Handle());
+    code.set_stackmaps(Object::null_array());
   } else {
     // Finalize the stack map array and add it to the code object.
     ASSERT(is_optimizing());
     code.set_stackmaps(
         Array::Handle(stackmap_table_builder_->FinalizeStackmaps(code)));
   }
 }
 
 
 void FlowGraphCompiler::FinalizeVarDescriptors(const Code& code) {
@@ skipping 607 matching lines @@
 
   for (int i = 0; i < len; i++) {
     sorted->Add(CidTarget(ic_data.GetReceiverClassIdAt(i),
                           &Function::ZoneHandle(ic_data.GetTargetAt(i)),
                           ic_data.GetCountAt(i)));
   }
   sorted->Sort(HighestCountFirst);
 }
 
 }  // namespace dart