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

runtime/vm/stub_code_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 "vm/globals.h"
 #if defined(TARGET_ARCH_MIPS)
 
 #include "vm/isolate.h"
 #include "vm/dart_entry.h"
 #include "vm/native_entry.h"
@@ skipping 36 matching lines @@
   ASSERT(context.isolate() == Isolate::Current());
   __ EnterDartFrame(0);
   __ LoadObject(CTX, context);
   __ PushObject(result);  // Push Null object for return value.
   __ PushObject(smi1);  // Push argument 1 smi1.
   __ PushObject(smi2);  // Push argument 2 smi2.
   ASSERT(kTestSmiSubRuntimeEntry.argument_count() == argc);
   __ CallRuntime(kTestSmiSubRuntimeEntry);  // Call SmiSub runtime func.
   __ addiu(SP, SP, Immediate(argc * kWordSize));
   __ Pop(V0);  // Pop return value from return slot.
-  __ LeaveDartFrame();
-  __ Ret();
+  __ LeaveDartFrameAndReturn();
 }
 
 
 TEST_CASE(CallRuntimeStubCode) {
   extern const Function& RegisterFakeFunction(const char* name,
                                               const Code& code);
   const int value1 = 10;
   const int value2 = 20;
   const char* kName = "Test_CallRuntimeStubCode";
   Assembler _assembler_;
@@ skipping 11 matching lines @@
 static void GenerateCallToCallLeafRuntimeStub(Assembler* assembler,
                                               int value1,
                                               int value2) {
   const Smi& smi1 = Smi::ZoneHandle(Smi::New(value1));
   const Smi& smi2 = Smi::ZoneHandle(Smi::New(value2));
   __ EnterDartFrame(0);
   __ ReserveAlignedFrameSpace(0);
   __ 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.
 }
 
 
 TEST_CASE(CallLeafRuntimeStubCode) {
   extern const Function& RegisterFakeFunction(const char* name,
                                               const Code& code);
   const int value1 = 10;
   const int value2 = 20;
   const char* kName = "Test_CallLeafRuntimeStubCode";
   Assembler _assembler_;
   GenerateCallToCallLeafRuntimeStub(&_assembler_, value1, value2);
   const Code& code = Code::Handle(Code::FinalizeCode(
       *CreateFunction("Test_CallLeafRuntimeStubCode"), &_assembler_));
   const Function& function = RegisterFakeFunction(kName, code);
   Smi& result = Smi::Handle();
   result ^= DartEntry::InvokeFunction(function, Object::empty_array());
   EXPECT_EQ((value1 + value2), result.Value());
 }
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_MIPS