Temporarily move vm_tag_ from BaseIsolate to Isolate.

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
===================================================================
--- runtime/vm/isolate.h	(revision 42877)
+++ runtime/vm/isolate.h	(working copy)
@@ -199,6 +199,12 @@
     return OFFSET_OF(Isolate, top_exit_frame_info_);
   }
 
+  uword vm_tag() const {
+    return vm_tag_;
+  }
+  void set_vm_tag(uword tag) {
+    vm_tag_ = tag;
+  }
   static intptr_t vm_tag_offset() {
     return OFFSET_OF(Isolate, vm_tag_);
   }
@@ -214,10 +220,6 @@
 
   TimerList& timer_list() { return timer_list_; }
 
-  static intptr_t current_zone_offset() {
-    return OFFSET_OF(Isolate, current_zone_);
-  }
-
   void set_init_callback_data(void* value) {
     init_callback_data_ = value;
   }
@@ -632,6 +634,7 @@
 
   static ThreadLocalKey isolate_key;
 
+  uword vm_tag_;
   StoreBuffer store_buffer_;
   ClassTable class_table_;
   MegamorphicCacheTable megamorphic_cache_table_;