Move reusable handles from isolate to thread.

runtime/vm/gc_marker.cc

Index: runtime/vm/gc_marker.cc
diff --git a/runtime/vm/gc_marker.cc b/runtime/vm/gc_marker.cc
index 2c53a0706161864cb90a52738164d42d01f9374b..44958d1dd052ef92f85a56dbd7544f40b34a6cb1 100644
--- a/runtime/vm/gc_marker.cc
+++ b/runtime/vm/gc_marker.cc
@@ -406,7 +406,7 @@ static bool IsUnreachable(const RawObject* raw_obj) {
 
 class MarkingWeakVisitor : public HandleVisitor {
  public:
-  MarkingWeakVisitor() : HandleVisitor(Isolate::Current()) {
+  MarkingWeakVisitor() : HandleVisitor(Thread::Current()) {
   }
 
   void VisitHandle(uword addr) {
@@ -414,7 +414,7 @@ class MarkingWeakVisitor : public HandleVisitor {
         reinterpret_cast<FinalizablePersistentHandle*>(addr);
     RawObject* raw_obj = handle->raw();
     if (IsUnreachable(raw_obj)) {
-      handle->UpdateUnreachable(isolate());
+      handle->UpdateUnreachable(thread()->isolate());
     }
   }