Fix SIMARM64 build on Mac.

runtime/vm/simulator_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 <math.h>  // for isnan.
 #include <setjmp.h>
 #include <stdlib.h>
 
 #include "vm/globals.h"
 #if defined(TARGET_ARCH_ARM64)
@@ skipping 229 matching lines @@
             if (Simulator::IsIllegalAddress(start)) {
               // If start isn't a valid address, warn and use PC instead
               OS::Print("First argument yields invalid address: 0x%"Px64"\n",
                         start);
               OS::Print("Using PC instead");
               start = sim_->get_pc();
             }
             end = start + (10 * Instr::kInstrSize);
           }
         } else {
-          intptr_t length;
+          int64_t length;
           if (GetValue(arg1, &start) && GetValue(arg2, &length)) {
             if (Simulator::IsIllegalAddress(start)) {
               // If start isn't a valid address, warn and use PC instead
               OS::Print("First argument yields invalid address: 0x%"Px64"\n",
                         start);
               OS::Print("Using PC instead\n");
               start = sim_->get_pc();
             }
             end = start + (length * Instr::kInstrSize);
           }
@@ skipping 1496 matching lines @@
   int64_t return_value;
   return_value = get_register(R0);
   return return_value;
 }
 
 }  // namespace dart
 
 #endif  // !defined(HOST_ARCH_ARM64)
 
 #endif  // defined TARGET_ARCH_ARM64