Remove unnecessary isolate argument from some visitors

runtime/vm/heap_test.cc

Index: runtime/vm/heap_test.cc
diff --git a/runtime/vm/heap_test.cc b/runtime/vm/heap_test.cc
index c76297d6d2b99a97e36fc7d88a49e701513580a6..e6b766ff8192fc731abbb1d439eaf8e5c0f3e777 100644
--- a/runtime/vm/heap_test.cc
+++ b/runtime/vm/heap_test.cc
@@ -236,8 +236,7 @@ TEST_CASE(ArrayHeapStats) {
 
 class FindOnly : public FindObjectVisitor {
  public:
-  FindOnly(Isolate* isolate, RawObject* target)
-      : FindObjectVisitor(isolate), target_(target) {
+  explicit FindOnly(RawObject* target) : target_(target) {
 #if defined(DEBUG)
     EXPECT_GT(Thread::Current()->no_safepoint_scope_depth(), 0);
 #endif
@@ -254,7 +253,7 @@ class FindOnly : public FindObjectVisitor {
 
 class FindNothing : public FindObjectVisitor {
  public:
-  FindNothing() : FindObjectVisitor(Isolate::Current()) { }
+  FindNothing() { }
   virtual ~FindNothing() { }
   virtual bool FindObject(RawObject* obj) const { return false; }
 };
@@ -268,7 +267,7 @@ TEST_CASE(FindObject) {
     const String& obj = String::Handle(String::New("x", spaces[space]));
     {
       NoSafepointScope no_safepoint;
-      FindOnly find_only(isolate, obj.raw());
+      FindOnly find_only(obj.raw());
       EXPECT(obj.raw() == heap->FindObject(&find_only));
     }
   }