Clarify heap write protection interface and use WritableVMIsolateScope to resolve issue 23217.

runtime/vm/pages.h

 // Copyright (c) 2011, 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_PAGES_H_
 #define VM_PAGES_H_
 
 #include "vm/freelist.h"
 #include "vm/globals.h"
 #include "vm/lockers.h"
@@ skipping 259 matching lines @@
 
   bool GrowthControlState() {
     return page_space_controller_.is_enabled();
   }
 
   bool NeedsExternalGC() const {
     return (max_external_in_words_ != 0) &&
         (ExternalInWords() > max_external_in_words_);
   }
 
-  // TODO(koda): Unify protection handling.
+  // Note: Code pages are made executable/non-executable when 'read_only' is
+  // true/false, respectively.
   void WriteProtect(bool read_only);
   void WriteProtectCode(bool read_only);
 
   void AddGCTime(int64_t micros) {
     gc_time_micros_ += micros;
   }
 
   int64_t gc_time_micros() const {
     return gc_time_micros_;
   }
@@ skipping 151 matching lines @@
   friend class HeapIterationScope;
   friend class PageSpaceController;
   friend class SweeperTask;
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(PageSpace);
 };
 
 }  // namespace dart
 
 #endif  // VM_PAGES_H_