Migrate NoSafepointScope; add constrained concurrent allocation to unit test

runtime/vm/object_graph.cc

Index: runtime/vm/object_graph.cc
diff --git a/runtime/vm/object_graph.cc b/runtime/vm/object_graph.cc
index 2e30f252907796eef24733c6c5ff08ea0e9432a5..879e87ca07c1702aa69f9382b19ffeaef951b606 100644
--- a/runtime/vm/object_graph.cc
+++ b/runtime/vm/object_graph.cc
@@ -256,7 +256,7 @@ class RetainingPathVisitor : public ObjectGraph::Visitor {
   // We cannot use a GrowableObjectArray, since we must not trigger GC.
   RetainingPathVisitor(RawObject* obj, const Array& path)
       : obj_(obj), path_(path), length_(0) {
-    ASSERT(Isolate::Current()->no_safepoint_scope_depth() != 0);
+    ASSERT(Thread::Current()->no_safepoint_scope_depth() != 0);
   }
 
   intptr_t length() const { return length_; }
@@ -313,7 +313,7 @@ class InboundReferencesVisitor : public ObjectVisitor,
                            Object* scratch)
     : ObjectVisitor(isolate), ObjectPointerVisitor(isolate), source_(NULL),
       target_(target), references_(references), scratch_(scratch), length_(0) {
-    ASSERT(Isolate::Current()->no_safepoint_scope_depth() != 0);
+    ASSERT(Thread::Current()->no_safepoint_scope_depth() != 0);
   }
 
   intptr_t length() const { return length_; }