Avoid caching thread in parser.

runtime/vm/parser.h

Index: runtime/vm/parser.h
===================================================================
--- runtime/vm/parser.h	(revision 44851)
+++ runtime/vm/parser.h	(working copy)
@@ -41,7 +41,7 @@
 class ParsedFunction : public ZoneAllocated {
  public:
   ParsedFunction(Thread* thread, const Function& function)
-      : thread_(thread),
+      : isolate_(thread->isolate()),
         function_(function),
         code_(Code::Handle(zone(), function.unoptimized_code())),
         node_sequence_(NULL),
@@ -150,12 +150,11 @@
   void record_await() { have_seen_await_expr_ = true; }
   bool have_seen_await() const { return have_seen_await_expr_; }
 
-  Thread* thread() const { return thread_; }
-  Isolate* isolate() const { return thread()->isolate(); }
-  Zone* zone() const { return thread()->zone(); }
+  Isolate* isolate() const { return isolate_; }
+  Zone* zone() const { return isolate()->current_zone(); }
 
  private:
-  Thread* thread_;
+  Isolate* isolate_;
   const Function& function_;
   Code& code_;
   SequenceNode* node_sequence_;
@@ -797,11 +796,10 @@
 
   RawInstance* TryCanonicalize(const Instance& instance, intptr_t token_pos);
 
-  Thread* thread() const { return thread_; }
-  Isolate* isolate() const { return thread()->isolate(); }
-  Zone* zone() const { return thread()->zone(); }
+  Isolate* isolate() const { return isolate_; }
+  Zone* zone() const { return isolate()->current_zone(); }
 
-  Thread* thread_;  // Cached current thread.
+  Isolate* isolate_;  // Cached current isolate.
 
   Script& script_;
   TokenStream::Iterator tokens_iterator_;