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

runtime/vm/heap.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/heap.h"
 
 #include "platform/assert.h"
 #include "platform/utils.h"
 #include "vm/flags.h"
 #include "vm/isolate.h"
@@ skipping 761 matching lines @@
     current_growth_controller_state_ = heap->GrowthControlState();
     heap->DisableGrowthControl();
 }
 
 
 NoHeapGrowthControlScope::~NoHeapGrowthControlScope() {
     Heap* heap = reinterpret_cast<Isolate*>(isolate())->heap();
     heap->SetGrowthControlState(current_growth_controller_state_);
 }
 
+
+WritableVMIsolateScope::WritableVMIsolateScope(Thread* thread)
+    : StackResource(thread) {
+  Dart::vm_isolate()->heap()->WriteProtect(false);
+}
+
+
+WritableVMIsolateScope::~WritableVMIsolateScope() {
+  ASSERT(Dart::vm_isolate()->heap()->UsedInWords(Heap::kNew) == 0);
+  Dart::vm_isolate()->heap()->WriteProtect(true);
+}
+
 }  // namespace dart