Clean up Isolate usages in ast visitor and hydrogen.

src/ast.h

Index: src/ast.h
diff --git a/src/ast.h b/src/ast.h
index 19d3580c4bf48dca3779864922ae2702ab843a87..ea365073823846d3dfa4ff19dd819a320a93bfd9 100644
--- a/src/ast.h
+++ b/src/ast.h
@@ -2180,7 +2180,7 @@ class RegExpEmpty: public RegExpTree {
 
 class AstVisitor BASE_EMBEDDED {
  public:
-  AstVisitor() : stack_overflow_(false) { }
+  AstVisitor() : isolate_(Isolate::Current()), stack_overflow_(false) { }
   virtual ~AstVisitor() { }
 
   // Stack overflow check and dynamic dispatch.
@@ -2210,7 +2210,11 @@ class AstVisitor BASE_EMBEDDED {
   AST_NODE_LIST(DEF_VISIT)
 #undef DEF_VISIT
 
+ protected:
+  Isolate* isolate() { return isolate_; }
+
  private:
+  Isolate* isolate_;
   bool stack_overflow_;
 };