Get rid of deprecated methods accessing mutator_thread_ instead of current thread

runtime/vm/allocation_test.cc

Index: runtime/vm/allocation_test.cc
diff --git a/runtime/vm/allocation_test.cc b/runtime/vm/allocation_test.cc
index f8fe5a04fcbc7fae0badf7918f026fd193f8a687..63b91f6c5aa1b0a407b9aeeb4a374eb96bbb09bc 100644
--- a/runtime/vm/allocation_test.cc
+++ b/runtime/vm/allocation_test.cc
@@ -32,7 +32,7 @@ class TestValueObject : public ValueObject {
 class TestStackResource : public StackResource {
  public:
   explicit TestStackResource(int* ptr)
-      : StackResource(Isolate::Current()), ptr_(ptr) {
+      : StackResource(Thread::Current()), ptr_(ptr) {
     EXPECT_EQ(1, *ptr_);
     *ptr_ = 2;
   }
@@ -53,7 +53,7 @@ class TestStackResource : public StackResource {
 class TestStackedStackResource : public StackResource {
  public:
   explicit TestStackedStackResource(int* ptr)
-      : StackResource(Isolate::Current()), ptr_(ptr) {
+      : StackResource(Thread::Current()), ptr_(ptr) {
     EXPECT_EQ(3, *ptr_);
     *ptr_ = 4;
   }