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

runtime/vm/assembler_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/assembler.h"
 #include "vm/os.h"
 #include "vm/unit_test.h"
@@ skipping 1042 matching lines @@
   typedef int (*SimpleCode)();
   EXPECT_EQ(42, EXECUTE_TEST_CODE_INT32(SimpleCode, test->entry()));
 }
 
 
 ASSEMBLER_TEST_GENERATE(AddOverflow_detect, assembler) {
   Register left = T0;
   Register right = T1;
   Register result = T2;
   Register overflow = T3;
+  Register scratch = T4;
   Label error, done;
 
   __ LoadImmediate(V0, 1);  // Success value.
 
   __ LoadImmediate(left, 0x7fffffff);
   __ LoadImmediate(right, 1);
   __ AdduDetectOverflow(result, left, right, overflow);
   __ bgez(overflow, &error);  // INT_MAX + 1 overflows.
 
   __ LoadImmediate(left, 0x7fffffff);
@@ skipping 14 matching lines @@
   __ LoadImmediate(right, 654321);
   __ AdduDetectOverflow(result, left, right, overflow);
   __ bltz(overflow, &error);  // 123456 + 654321 does not overflow.
 
   __ LoadImmediate(left, 0x80000000);
   __ LoadImmediate(right, -1);
   __ AdduDetectOverflow(result, left, right, overflow);
   __ bgez(overflow, &error);  // INT_MIN - 1 overflows.
 
   // result has 0x7fffffff.
-  __ AdduDetectOverflow(result, result, result, overflow);
+  __ AdduDetectOverflow(result, result, result, overflow, scratch);
   __ bgez(overflow, &error);  // INT_MAX + INT_MAX overflows.
 
   __ LoadImmediate(left, 0x80000000);
   __ LoadImmediate(right, 0x80000000);
   __ AdduDetectOverflow(result, left, right, overflow);
   __ bgez(overflow, &error);  // INT_MIN + INT_MIN overflows.
 
   __ LoadImmediate(left, -123456);
   __ LoadImmediate(right, -654321);
   __ AdduDetectOverflow(result, left, right, overflow);
@@ skipping 235 matching lines @@
                      A1);
   __ lw(RA, Address(SP, 0 * kWordSize));
   __ lw(CTX, Address(SP, 1 * kWordSize));
   __ addiu(SP, SP, Immediate(2 * kWordSize));
   __ Ret();
 }
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_MIPS