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

runtime/vm/object.cc

Index: runtime/vm/object.cc
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index 2366142b7a56dacf9e74fbdcb5ace7e723b849d9..20ba0722f7fb4c3fe3d0a151bbc325cbb17e00fb 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -1718,9 +1718,8 @@ void Object::InitializeObject(uword address,
                               intptr_t class_id,
                               intptr_t size,
                               bool is_vm_object) {
-  // TODO(iposva): Get a proper halt instruction from the assembler which
-  // would be needed here for code objects.
-  uword initial_value = reinterpret_cast<uword>(null_);
+  uword initial_value = (class_id == kInstructionsCid)
+      ? Assembler::GetBreakInstructionFiller() : reinterpret_cast<uword>(null_);
   uword cur = address;
   uword end = address + size;
   while (cur < end) {