Use Object::null_object() instead of Object::ZoneHandle() when a null object

runtime/vm/flow_graph_compiler_x64.cc

Index: runtime/vm/flow_graph_compiler_x64.cc
===================================================================
--- runtime/vm/flow_graph_compiler_x64.cc	(revision 37955)
+++ runtime/vm/flow_graph_compiler_x64.cc	(working copy)
@@ -592,7 +592,7 @@
     // Generate runtime call.
     __ movq(RDX, Address(RSP, 0));  // Get instantiator type arguments.
     __ movq(RCX, Address(RSP, kWordSize));  // Get instantiator.
-    __ PushObject(Object::ZoneHandle(), PP);  // Make room for the result.
+    __ PushObject(Object::null_object(), PP);  // Make room for the result.
     __ pushq(RAX);  // Push the instance.
     __ PushObject(type, PP);  // Push the type.
     __ pushq(RCX);  // TODO(srdjan): Pass instantiator instead of null.
@@ -662,7 +662,7 @@
 
   // Generate throw new TypeError() if the type is malformed or malbounded.
   if (dst_type.IsMalformedOrMalbounded()) {
-    __ PushObject(Object::ZoneHandle(), PP);  // Make room for the result.
+    __ PushObject(Object::null_object(), PP);  // Make room for the result.
     __ pushq(RAX);  // Push the source object.
     __ PushObject(dst_name, PP);  // Push the name of the destination.
     __ PushObject(dst_type, PP);  // Push the type of the destination.
@@ -688,7 +688,7 @@
   __ Bind(&runtime_call);
   __ movq(RDX, Address(RSP, 0));  // Get instantiator type arguments.
   __ movq(RCX, Address(RSP, kWordSize));  // Get instantiator.
-  __ PushObject(Object::ZoneHandle(), PP);  // Make room for the result.
+  __ PushObject(Object::null_object(), PP);  // Make room for the result.
   __ pushq(RAX);  // Push the source object.
   __ PushObject(dst_type, PP);  // Push the type of the destination.
   __ pushq(RCX);  // Instantiator.