VM: Initialize Instruction objects with break instructions instead of null.

runtime/vm/assembler_x64.h

 // 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.
 
 #ifndef VM_ASSEMBLER_X64_H_
 #define VM_ASSEMBLER_X64_H_
 
 #ifndef VM_ASSEMBLER_H_
 #error Do not include assembler_x64.h directly; use assembler.h instead.
 #endif
@@ skipping 666 matching lines @@
   void ffree(intptr_t value);
 
   void fsin();
   void fcos();
 
   // 'size' indicates size in bytes and must be in the range 1..8.
   void nop(int size = 1);
   void int3();
   void hlt();
 
+  static uword GetBreakInstructionFiller() {
+    return 0xCCCCCCCCCCCCCCCC;
+  }
+
   // Note: verified_mem mode forces far jumps.
   void j(Condition condition, Label* label, bool near = kFarJump);
 
   void jmp(Register reg);
   void jmp(const Address& address);
   // Note: verified_mem mode forces far jumps.
   void jmp(Label* label, bool near = kFarJump);
   void jmp(const ExternalLabel* label);
   void jmp(const StubEntry& stub_entry);
 
@@ skipping 497 matching lines @@
 }
 
 
 inline void Assembler::EmitOperandSizeOverride() {
   EmitUint8(0x66);
 }
 
 }  // namespace dart
 
 #endif  // VM_ASSEMBLER_X64_H_