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

runtime/vm/pages.cc

 // 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.
 
 #include "vm/pages.h"
 
 #include "platform/assert.h"
 #include "vm/compiler_stats.h"
 #include "vm/gc_marker.h"
 #include "vm/gc_sweeper.h"
@@ skipping 118 matching lines @@
 
 void HeapPage::WriteProtect(bool read_only) {
   VirtualMemory::Protection prot;
   if (read_only) {
     if (executable_) {
       prot = VirtualMemory::kReadExecute;
     } else {
       prot = VirtualMemory::kReadOnly;
     }
   } else {
-    // TODO(23217): Should this really make all pages non-executable?
     prot = VirtualMemory::kReadWrite;
   }
   bool status = memory_->Protect(prot);
   ASSERT(status);
 }
 
 
 PageSpace::PageSpace(Heap* heap,
                      intptr_t max_capacity_in_words,
                      intptr_t max_external_in_words)
@@ skipping 1030 matching lines @@
     return 0;
   } else {
     ASSERT(total_time >= gc_time);
     int result = static_cast<int>((static_cast<double>(gc_time) /
                                    static_cast<double>(total_time)) * 100);
     return result;
   }
 }
 
 }  // namespace dart