Migrate most uses of Isolate::current_zone to Thread::zone.

runtime/vm/service.cc

Index: runtime/vm/service.cc
diff --git a/runtime/vm/service.cc b/runtime/vm/service.cc
index c40f71e388364a458220b38c453ce11ef2447c8e..4cbc5ddba46f142d49aa4c71cb4551b87d1c3ebb 100644
--- a/runtime/vm/service.cc
+++ b/runtime/vm/service.cc
@@ -1168,7 +1168,7 @@ static RawObject* LookupHeapObjectCode(Isolate* isolate,
 static RawObject* LookupHeapObject(Isolate* isolate,
                                    const char* id_original,
                                    ObjectIdRing::LookupResult* result) {
-  char* id = isolate->current_zone()->MakeCopyOfString(id_original);
+  char* id = Thread::Current()->zone()->MakeCopyOfString(id_original);
 
   // Parse the id by splitting at each '/'.
   const int MAX_PARTS = 8;
@@ -1296,7 +1296,7 @@ static bool PrintMessage(JSONStream* js, Isolate* isolate, const char* id) {
       MessageSnapshotReader reader(message->data(),
                                    message->len(),
                                    isolate,
-                                   isolate->current_zone());
+                                   Thread::Current()->zone());
       const Object& msg_obj = Object::Handle(reader.ReadObject());
       msg_obj.PrintJSON(js);
       return true;