Specialize breakpoint stubs by set of live registers of the stubs they are intercepting.

runtime/vm/intermediate_language_arm64.cc

 // Copyright (c) 2014, 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_ARM64.
 #if defined(TARGET_ARCH_ARM64)
 
 #include "vm/intermediate_language.h"
 
 #include "vm/dart_entry.h"
@@ skipping 5321 matching lines @@
                          &label,
                          PcDescriptors::kOther,
                          locs());
   __ Drop(ArgumentCount());  // Discard arguments.
 }
 
 
 void DebugStepCheckInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
   ASSERT(!compiler->is_optimizing());
   const ExternalLabel label(StubCode::DebugStepCheckEntryPoint());
-  __ LoadImmediate(R4, 0, kNoPP);
-  __ LoadImmediate(R5, 0, kNoPP);
   compiler->GenerateCall(token_pos(), &label, stub_kind_, locs());
 #if defined(DEBUG)
   __ LoadImmediate(R4, kInvalidObjectPointer, kNoPP);
   __ LoadImmediate(R5, kInvalidObjectPointer, kNoPP);
 #endif
 }
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_ARM64