Get rid of deprecated methods accessing mutator_thread_ instead of current thread

runtime/vm/base_isolate.h

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_BASE_ISOLATE_H_
 #define VM_BASE_ISOLATE_H_
 
 #include "platform/assert.h"
 #include "vm/globals.h"
 
@@ skipping 24 matching lines @@
       : mutator_thread_(NULL) {
   }
 
   ~BaseIsolate() {
     // Do not delete stack resources: top_resource_ and current_zone_.
   }
 
   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);
 };
 
 }  // namespace dart
 
 #endif  // VM_BASE_ISOLATE_H_