Cache the isolate pointer in the parser object.

runtime/vm/parser.h

Index: runtime/vm/parser.h
===================================================================
--- runtime/vm/parser.h	(revision 23504)
+++ runtime/vm/parser.h	(working copy)
@@ -17,6 +17,7 @@
 
 // Forward declarations.
 class Function;
+class Isolate;
 class LiteralToken;
 class Script;
 class TokenStream;
@@ -622,6 +623,10 @@
       const Function& constructor,
       ArgumentListNode* arguments);
 
+  Isolate* isolate() const { return isolate_; }
+
+  Isolate* isolate_;  // Cached current isolate.
+
   Script& script_;
   TokenStream::Iterator tokens_iterator_;
   Token::Kind token_kind_;  // Cached token kind for current token.