Take ast node id counting away from Isolate.

src/zone.h

Index: src/zone.h
diff --git a/src/zone.h b/src/zone.h
index a690b8d8caf1bb6ec3144908f37b4e8736c982d8..6f552b6524413c08172d25e6fd8c70ca2b27f85a 100644
--- a/src/zone.h
+++ b/src/zone.h
@@ -63,9 +63,9 @@ class Zone {
 
   inline void adjust_segment_bytes_allocated(int delta);
 
-  inline unsigned allocation_size() { return allocation_size_; }
+  inline unsigned allocation_size() const { return allocation_size_; }
 
-  inline Isolate* isolate() { return isolate_; }
+  inline Isolate* isolate() const { return isolate_; }
 
  private:
   friend class Isolate;