Remove more uses of Isolate::current_zone.

runtime/vm/parser.h

Index: runtime/vm/parser.h
diff --git a/runtime/vm/parser.h b/runtime/vm/parser.h
index 9cab860ca46fa309c9d15980512bc09d7d499d97..92eb87d7ef6079a12f68b731bcd21d79edf1594b 100644
--- a/runtime/vm/parser.h
+++ b/runtime/vm/parser.h
@@ -41,7 +41,7 @@ struct TopLevel;
 class ParsedFunction : public ZoneAllocated {
  public:
   ParsedFunction(Thread* thread, const Function& function)
-      : isolate_(thread->isolate()),
+      : thread_(thread),
         function_(function),
         code_(Code::Handle(zone(), function.unoptimized_code())),
         node_sequence_(NULL),
@@ -150,11 +150,11 @@ class ParsedFunction : public ZoneAllocated {
   void record_await() { have_seen_await_expr_ = true; }
   bool have_seen_await() const { return have_seen_await_expr_; }
 
-  Isolate* isolate() const { return isolate_; }
-  Zone* zone() const { return isolate()->current_zone(); }
+  Isolate* isolate() const { return thread_->isolate(); }
+  Zone* zone() const { return thread_->zone(); }
 
  private:
-  Isolate* isolate_;
+  Thread* thread_;
   const Function& function_;
   Code& code_;
   SequenceNode* node_sequence_;
@@ -807,9 +807,10 @@ class Parser : public ValueObject {
   RawInstance* TryCanonicalize(const Instance& instance, intptr_t token_pos);
 
   Isolate* isolate() const { return isolate_; }
-  Zone* zone() const { return isolate()->current_zone(); }
+  Zone* zone() const { return thread_->zone(); }
 
   Isolate* isolate_;  // Cached current isolate.
+  Thread* thread_;
 
   Script& script_;
   TokenStream::Iterator tokens_iterator_;