Fix profiler access to the Dart stack pointer on ARM64.

runtime/vm/signal_handler_macos.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"
+#include "vm/instructions.h"
 #include "vm/simulator.h"
 #include "vm/signal_handler.h"
 #if defined(TARGET_OS_MACOS)
 
 namespace dart {
 
 uintptr_t SignalHandler::GetProgramCounter(const mcontext_t& mcontext) {
   uintptr_t pc = 0;
 
 #if defined(HOST_ARCH_IA32)
@@ skipping 45 matching lines @@
 #else
   UNIMPLEMENTED();
 #endif  // HOST_ARCH_...
 
   return sp;
 }
 
 
 uintptr_t SignalHandler::GetDartStackPointer(const mcontext_t& mcontext) {
 #if defined(TARGET_ARCH_ARM64) && !defined(USING_SIMULATOR)
-  return static_cast<uintptr_t>(mcontext->__ss.__x[19]);
+  return static_cast<uintptr_t>(mcontext->__ss.__x[SPREG]);
 #else
   return GetCStackPointer(mcontext);
 #endif
 }
 
 
 uintptr_t SignalHandler::GetLinkRegister(const mcontext_t& mcontext) {
   uintptr_t lr = 0;
 
 #if defined(TARGET_ARCH_IA32)
@@ skipping 39 matching lines @@
   sigemptyset(&act.sa_mask);
   act.sa_flags = 0;
   int r = sigaction(SIGPROF, &act, NULL);
   ASSERT(r == 0);
 }
 
 
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_MACOS)