Use a breakpoint instruction for a stop message on arm instead of

runtime/vm/simulator_arm64.cc

Index: runtime/vm/simulator_arm64.cc
===================================================================
--- runtime/vm/simulator_arm64.cc	(revision 45318)
+++ runtime/vm/simulator_arm64.cc	(working copy)
@@ -1689,22 +1689,23 @@
              (instr->Bits(21, 3) == 1)) {
     // Format(instr, "brk 'imm16");
     SimulatorDebugger dbg(this);
-    uint16_t imm = static_cast<uint16_t>(instr->Imm16Field());
-    char buffer[32];
-    snprintf(buffer, sizeof(buffer), "brk #0x%x", imm);
-    set_pc(get_pc() + Instr::kInstrSize);
-    dbg.Stop(instr, buffer);
-  } else if ((instr->Bits(0, 2) == 0) && (instr->Bits(2, 3) == 0) &&
-             (instr->Bits(21, 3) == 2)) {
-    // Format(instr, "hlt 'imm16");
-    uint16_t imm = static_cast<uint16_t>(instr->Imm16Field());
+    int32_t imm = instr->Imm16Field();
     if (imm == Instr::kStopMessageCode) {
-      SimulatorDebugger dbg(this);
       const char* message = *reinterpret_cast<const char**>(
           reinterpret_cast<intptr_t>(instr) - 2 * Instr::kInstrSize);
       set_pc(get_pc() + Instr::kInstrSize);
       dbg.Stop(instr, message);
-    } else if (imm == Instr::kSimulatorBreakCode) {
+    } else {
+      char buffer[32];
+      snprintf(buffer, sizeof(buffer), "brk #0x%x", imm);
+      set_pc(get_pc() + Instr::kInstrSize);
+      dbg.Stop(instr, buffer);
+    }
+  } else if ((instr->Bits(0, 2) == 0) && (instr->Bits(2, 3) == 0) &&
+             (instr->Bits(21, 3) == 2)) {
+    // Format(instr, "hlt 'imm16");
+    uint16_t imm = static_cast<uint16_t>(instr->Imm16Field());
+    if (imm == Instr::kSimulatorBreakCode) {
       SimulatorDebugger dbg(this);
       dbg.Stop(instr, "breakpoint");
     } else if (imm == Instr::kSimulatorRedirectCode) {