Avoid TLS load in AstNode constructor.

src/ast.h

Index: src/ast.h
diff --git a/src/ast.h b/src/ast.h
index 27d82cb34139c5efaed5786a6ca58bbea30c86f3..e9003a104f2cbfdf06dd10449e4aae6a8f2dcdf7 100644
--- a/src/ast.h
+++ b/src/ast.h
@@ -134,11 +134,15 @@ class AstNode: public ZoneObject {
   static const int kNoNumber = -1;
   static const int kFunctionEntryId = 2;  // Using 0 could disguise errors.
 
-  AstNode() {
-    Isolate* isolate = Isolate::Current();
+  // Override ZoneObjects's new to count allocated AST nodes.
+  void* operator new(size_t size, Zone* zone) {
+    Isolate* isolate = zone->isolate();
     isolate->set_ast_node_count(isolate->ast_node_count() + 1);
+    return zone->New(size);
   }
 
+  AstNode() {}
+
   virtual ~AstNode() { }
 
   virtual void Accept(AstVisitor* v) = 0;
@@ -173,6 +177,11 @@ class AstNode: public ZoneObject {
     return tmp;
   }
 
+ private:
+  // Hidden to prevent accidental usage. It would have to load the
+  // current zone from the TLS.
+  void* operator new(size_t size);
+
   friend class CaseClause;  // Generates AST IDs.
 };