On MIPS, keeps Object::null() in a register.

runtime/vm/object_mips_test.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 "platform/assert.h"
 #include "vm/globals.h"
 #if defined(TARGET_ARCH_MIPS)
 
 #include "vm/assembler.h"
 #include "vm/object.h"
@@ skipping 19 matching lines @@
 }
 
 
 // Generate a dart code sequence that embeds a string object in it.
 // This is used to test Embedded String objects in the instructions.
 void GenerateEmbedStringInCode(Assembler* assembler, const char* str) {
   __ EnterDartFrame(0);  // To setup pp.
   const String& string_object =
       String::ZoneHandle(String::New(str, Heap::kOld));
   __ LoadObject(V0, string_object);
-  __ LeaveDartFrame();
-  __ Ret();
+  __ LeaveDartFrameAndReturn();
 }
 
 
 // Generate a dart code sequence that embeds a smi object in it.
 // This is used to test Embedded Smi objects in the instructions.
 void GenerateEmbedSmiInCode(Assembler* assembler, intptr_t value) {
   // No need to setup pp, since Smis are not stored in the object pool.
   const Smi& smi_object = Smi::ZoneHandle(Smi::New(value));
   __ LoadObject(V0, smi_object);
   __ Ret();
 }
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_MIPS