Remove zapping of registers that has not turned up any GC bugs in a long time.

runtime/vm/intermediate_language_mips.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"  // Needed here to get TARGET_ARCH_MIPS.
 #if defined(TARGET_ARCH_MIPS)
 
 #include "vm/intermediate_language.h"
 
 #include "vm/dart_entry.h"
@@ skipping 5562 matching lines @@
   compiler->AddStubCallTarget(stub);
   __ Drop(ArgumentCount());  // Discard arguments.
 }
 
 
 void DebugStepCheckInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
   ASSERT(!compiler->is_optimizing());
   StubCode* stub_code = compiler->isolate()->stub_code();
   const ExternalLabel label(stub_code->DebugStepCheckEntryPoint());
   compiler->GenerateCall(token_pos(), &label, stub_kind_, locs());
-#if defined(DEBUG)
-  __ LoadImmediate(S4, kInvalidObjectPointer);
-  __ LoadImmediate(S5, kInvalidObjectPointer);
-#endif
 }
 
 
 LocationSummary* GrowRegExpStackInstr::MakeLocationSummary(
     Zone* zone, bool opt) const {
   const intptr_t kNumInputs = 1;
   const intptr_t kNumTemps = 0;
   LocationSummary* locs = new(zone) LocationSummary(
       zone, kNumInputs, kNumTemps, LocationSummary::kCall);
   locs->set_in(0, Location::RegisterLocation(T0));
@@ skipping 16 matching lines @@
                                 1,
                                 locs());
   __ lw(result, Address(SP, 1 * kWordSize));
   __ addiu(SP, SP, Immediate(2 * kWordSize));
 }
 
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_MIPS