Get rid of deprecated methods accessing mutator_thread_ instead of current thread

runtime/vm/base_isolate.h

Index: runtime/vm/base_isolate.h
diff --git a/runtime/vm/base_isolate.h b/runtime/vm/base_isolate.h
index 833f7e5ec7dbb7d35263530448288e8a7f5aece6..b2cc7fc223fcf04f0bc4bdcff03d2e219cf27825 100644
--- a/runtime/vm/base_isolate.h
+++ b/runtime/vm/base_isolate.h
@@ -42,9 +42,6 @@ class BaseIsolate {
   Thread* mutator_thread_;
 
  private:
-  // During migration, some deprecated interfaces will default to using the
-  // mutator_thread_ (can't use accessor in Isolate due to circular deps).
-  friend class StackResource;
   DISALLOW_COPY_AND_ASSIGN(BaseIsolate);
 };