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

runtime/vm/object.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/object.h"
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "vm/assembler.h"
 #include "vm/cpu.h"
@@ skipping 933 matching lines @@
   SET_CLASS_NAME(unhandled_exception, UnhandledException);
   SET_CLASS_NAME(unwind_error, UnwindError);
 
   // Set up names for object array and one byte string class which are
   // pre-allocated in the vm isolate also.
   cls = isolate->object_store()->array_class();
   cls.set_name(Symbols::_List());
   cls = isolate->object_store()->one_byte_string_class();
   cls.set_name(Symbols::OneByteString());
 
-  // Make the VM isolate read-only after setting all objects as marked.
-  PremarkingVisitor premarker(isolate);
-  isolate->heap()->WriteProtect(false);
-  ASSERT(isolate->heap()->UsedInWords(Heap::kNew) == 0);
-  isolate->heap()->IterateOldObjects(&premarker);
-  isolate->heap()->WriteProtect(true);
+  {
+    ASSERT(isolate == Dart::vm_isolate());
+    WritableVMIsolateScope scope(Thread::Current());
+    PremarkingVisitor premarker(isolate);
+    ASSERT(isolate->heap()->UsedInWords(Heap::kNew) == 0);
+    isolate->heap()->IterateOldObjects(&premarker);
+    // Make the VM isolate read-only again after setting all objects as marked.
+  }
 }
 
 
 void Object::InitVmIsolateSnapshotObjectTable(intptr_t len) {
   ASSERT(Isolate::Current() == Dart::vm_isolate());
   *vm_isolate_snapshot_object_table_ = Array::New(len, Heap::kOld);
 }
 
 
 // Make unused space in an object whose type has been transformed safe
@@ skipping 20474 matching lines @@
   return tag_label.ToCString();
 }
 
 
 void UserTag::PrintJSONImpl(JSONStream* stream, bool ref) const {
   Instance::PrintJSONImpl(stream, ref);
 }
 
 
 }  // namespace dart