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

runtime/vm/flow_graph_compiler.cc

Index: runtime/vm/flow_graph_compiler.cc
===================================================================
--- runtime/vm/flow_graph_compiler.cc	(revision 24241)
+++ runtime/vm/flow_graph_compiler.cc	(working copy)
@@ -467,7 +467,7 @@
 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());