Address Siva's comments from https://codereview.chromium.org/1877973002/

runtime/vm/isolate.h

 // Copyright (c) 2013, 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_ISOLATE_H_
 #define VM_ISOLATE_H_
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "vm/atomic.h"
@@ skipping 507 matching lines @@
       AtomicOperations::IncrementBy(&loading_invalidation_gen_, 1);
     }
   }
   intptr_t loading_invalidation_gen() {
     return AtomicOperations::LoadRelaxedIntPtr(&loading_invalidation_gen_);
   }
 
   // Used by mutator thread to notify background compiler which fields
   // triggered code invalidation.
   void AddDisablingField(const Field& field);
-  // Returns Field::null() if none available in the list.
+  // Returns Field::null() if none available in the list. Can be called
+  // only from background compiler and while mutator thread is at safepoint.
   RawField* GetDisablingField();
   void ClearDisablingFieldList();
 
   // Used by background compiler which field became boxed and must trigger
   // deoptimization in the mutator thread.
   void AddDeoptimizingBoxedField(const Field& field);
   // Returns Field::null() if none available in the list.
   RawField* GetDeoptimizingBoxedField();
 
   RawObject* InvokePendingServiceExtensionCalls();
@@ skipping 384 matching lines @@
   intptr_t* spawn_count_;
 
   Dart_IsolateFlags isolate_flags_;
   bool paused_;
   bool errors_are_fatal_;
 };
 
 }  // namespace dart
 
 #endif  // VM_ISOLATE_H_