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

runtime/vm/assembler_x64.h

Index: runtime/vm/assembler_x64.h
diff --git a/runtime/vm/assembler_x64.h b/runtime/vm/assembler_x64.h
index e0ac23f953eaeea5ee6d8fe0befad8244a2b65fa..89b294497c32a69d2eb553b63f3f03abed3e1187 100644
--- a/runtime/vm/assembler_x64.h
+++ b/runtime/vm/assembler_x64.h
@@ -684,6 +684,10 @@ class Assembler : public ValueObject {
   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);