Remove unnecessary isolate argument from some visitors

runtime/vm/verifier.h

Index: runtime/vm/verifier.h
diff --git a/runtime/vm/verifier.h b/runtime/vm/verifier.h
index 99b605610b03022c107e586accd7441ef2d163b2..a34d7f6305ff0939bce8a3ac741c1dde6aeb5f7e 100644
--- a/runtime/vm/verifier.h
+++ b/runtime/vm/verifier.h
@@ -30,7 +30,7 @@ class VerifyObjectVisitor : public ObjectVisitor {
   VerifyObjectVisitor(Isolate* isolate,
                      ObjectSet* allocated_set,
                      MarkExpectation mark_expectation)
-      : ObjectVisitor(isolate),
+      : isolate_(isolate),
         allocated_set_(allocated_set),
         mark_expectation_(mark_expectation) {
   }
@@ -38,6 +38,7 @@ class VerifyObjectVisitor : public ObjectVisitor {
   virtual void VisitObject(RawObject* obj);
 
  private:
+  Isolate* isolate_;
   ObjectSet* allocated_set_;
   MarkExpectation mark_expectation_;