Add IsMutatorThread to the Thread class and use it instead of MutatorThreadIsCurrentThread

runtime/vm/thread.cc

Index: runtime/vm/thread.cc
diff --git a/runtime/vm/thread.cc b/runtime/vm/thread.cc
index a2af102a7ebc4736a645e2fd800e2c5b4ef5aa8a..076031063d2361f5a05639563ef7765df7cd4a74 100644
--- a/runtime/vm/thread.cc
+++ b/runtime/vm/thread.cc
@@ -341,7 +341,7 @@ void Thread::EnterIsolateAsHelper(Isolate* isolate, bool bypass_safepoint) {
   ASSERT(thread->thread_interrupt_data_ == NULL);
   // Do not update isolate->mutator_thread, but perform sanity check:
   // this thread should not be both the main mutator and helper.
-  ASSERT(!isolate->MutatorThreadIsCurrentThread());
+  ASSERT(!thread->IsMutatorThread());
   thread->Schedule(isolate, bypass_safepoint);
 }
 
@@ -355,7 +355,7 @@ void Thread::ExitIsolateAsHelper(bool bypass_safepoint) {
   thread->StoreBufferRelease();
   thread->isolate_ = NULL;
   thread->heap_ = NULL;
-  ASSERT(!isolate->MutatorThreadIsCurrentThread());
+  ASSERT(!thread->IsMutatorThread());
 }
 
 
@@ -406,6 +406,11 @@ void Thread::StoreBufferAcquire() {
 }
 
 
+bool Thread::IsMutatorThread() const {
+  return ((isolate_ != NULL) && (isolate_->mutator_thread() == this));
+}
+
+
 CHA* Thread::cha() const {
   ASSERT(isolate_ != NULL);
   return cha_;