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

runtime/vm/dart_entry.cc

Index: runtime/vm/dart_entry.cc
diff --git a/runtime/vm/dart_entry.cc b/runtime/vm/dart_entry.cc
index 2ce752da3d2d047a7fbd138a0d83a8b2964a5633..5b6e28f85f42df985091dcb799b2c9014419941c 100644
--- a/runtime/vm/dart_entry.cc
+++ b/runtime/vm/dart_entry.cc
@@ -84,7 +84,7 @@ RawObject* DartEntry::InvokeFunction(const Function& function,
   Thread* thread = Thread::Current();
   Zone* zone = thread->zone();
   Isolate* isolate = thread->isolate();
-  ASSERT(isolate->MutatorThreadIsCurrentThread());
+  ASSERT(thread->IsMutatorThread());
   if (!function.HasCode()) {
     const Error& error = Error::Handle(
         zone, Compiler::CompileFunction(thread, function));