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

runtime/vm/snapshot.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/snapshot.h"
 
 #include "platform/assert.h"
 #include "vm/bootstrap.h"
 #include "vm/class_finalizer.h"
 #include "vm/dart.h"
@@ skipping 1990 matching lines @@
     writer.FillHeader(writer.kind());
     writer.UnmarkAll();
 
     isolate_snapshot_size_ = writer.BytesWritten();
   } else {
     writer.ThrowException(writer.exception_type(), writer.exception_msg());
   }
 }
 
 
-class WritableVMIsolateScope : StackResource {
- public:
-  explicit WritableVMIsolateScope(Thread* thread) : StackResource(thread) {
-    Dart::vm_isolate()->heap()->WriteProtect(false);
-  }
-
-  ~WritableVMIsolateScope() {
-    ASSERT(Dart::vm_isolate()->heap()->UsedInWords(Heap::kNew) == 0);
-    Dart::vm_isolate()->heap()->WriteProtect(true);
-  }
-};
-
-
 void FullSnapshotWriter::WriteFullSnapshot() {
   if (!vm_isolate_is_symbolic_) {
     // TODO(asiva): Don't mutate object headers during serialization.
     WritableVMIsolateScope scope(Thread::Current());
 
     if (vm_isolate_snapshot_buffer() != NULL) {
       WriteVmIsolateSnapshot();
     }
     WriteIsolateFullSnapshot();
 
@@ skipping 770 matching lines @@
     NoSafepointScope no_safepoint;
     WriteObject(obj.raw());
     UnmarkAll();
   } else {
     ThrowException(exception_type(), exception_msg());
   }
 }
 
 
 }  // namespace dart