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

runtime/vm/assembler_arm64.h

 // 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.
 
 #ifndef VM_ASSEMBLER_ARM64_H_
 #define VM_ASSEMBLER_ARM64_H_
 
 #ifndef VM_ASSEMBLER_H_
 #error Do not include assembler_arm64.h directly; use assembler.h instead.
 #endif
@@ skipping 809 matching lines @@
   void br(Register rn) {
     EmitUnconditionalBranchRegOp(BR, rn);
   }
   void blr(Register rn) {
     EmitUnconditionalBranchRegOp(BLR, rn);
   }
   void ret(Register rn = R30) {
     EmitUnconditionalBranchRegOp(RET, rn);
   }
 
-  // Exceptions.
-  void hlt(uint16_t imm) {
-    EmitExceptionGenOp(HLT, imm);
-  }
+  // Breakpoint.
   void brk(uint16_t imm) {
     EmitExceptionGenOp(BRK, imm);
   }
 
   // Double floating point.
   bool fmovdi(VRegister vd, double immd) {
     int64_t imm64 = bit_cast<int64_t, double>(immd);
     const uint8_t bit7 = imm64 >> 63;
     const uint8_t bit6 = (~(imm64 >> 62)) & 0x1;
     const uint8_t bit54 = (imm64 >> 52) & 0x3;
@@ skipping 1080 matching lines @@
                                   Register value,
                                   Label* no_update);
 
   DISALLOW_ALLOCATION();
   DISALLOW_COPY_AND_ASSIGN(Assembler);
 };
 
 }  // namespace dart
 
 #endif  // VM_ASSEMBLER_ARM64_H_