Move vm_tags from isolate to thread, since we may have multiple threads in same isolate (GC, backgr…

runtime/vm/tags.cc

Index: runtime/vm/tags.cc
diff --git a/runtime/vm/tags.cc b/runtime/vm/tags.cc
index 4224a229de76a953ddf7bfe5512010291d4e9d52..4349844158ebd48805d515954a224a2f2760c8d1 100644
--- a/runtime/vm/tags.cc
+++ b/runtime/vm/tags.cc
@@ -84,16 +84,16 @@ VMTag::TagEntry VMTag::entries_[] = {
 VMTagScope::VMTagScope(Thread* thread, uword tag, bool conditional_set)
     : StackResource(thread) {
   ASSERT(isolate() != NULL);
-  previous_tag_ = isolate()->vm_tag();
+  previous_tag_ = thread->vm_tag();
   if (conditional_set) {
-    isolate()->set_vm_tag(tag);
+    thread->set_vm_tag(tag);
   }
 }
 
 
 VMTagScope::~VMTagScope() {
   ASSERT(isolate() != NULL);
-  isolate()->set_vm_tag(previous_tag_);
+  thread()->set_vm_tag(previous_tag_);
 }