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

runtime/vm/stub_code_mips_test.cc

Index: runtime/vm/stub_code_mips_test.cc
===================================================================
--- runtime/vm/stub_code_mips_test.cc	(revision 22325)
+++ runtime/vm/stub_code_mips_test.cc	(working copy)
@@ -54,8 +54,7 @@
   __ CallRuntime(kTestSmiSubRuntimeEntry);  // Call SmiSub runtime func.
   __ addiu(SP, SP, Immediate(argc * kWordSize));
   __ Pop(V0);  // Pop return value from return slot.
-  __ LeaveDartFrame();
-  __ Ret();
+  __ LeaveDartFrameAndReturn();
 }
 
 
@@ -87,8 +86,7 @@
   __ LoadObject(A0, smi1);  // Set up argument 1 smi1.
   __ LoadObject(A1, smi2);  // Set up argument 2 smi2.
   __ CallRuntime(kTestLeafSmiAddRuntimeEntry);  // Call SmiAdd runtime func.
-  __ LeaveDartFrame();
-  __ Ret();  // Return value is in R0.
+  __ LeaveDartFrameAndReturn();  // Return value is in V0.
 }