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

runtime/vm/snapshot.cc

Index: runtime/vm/snapshot.cc
diff --git a/runtime/vm/snapshot.cc b/runtime/vm/snapshot.cc
index 5875720e09df701fb0bfdd1a4dc64775c5be295e..d83606c1ee795c4b25459fe86a7093b0a68129a3 100644
--- a/runtime/vm/snapshot.cc
+++ b/runtime/vm/snapshot.cc
@@ -2008,19 +2008,6 @@ void FullSnapshotWriter::WriteIsolateFullSnapshot() {
 }
 
 
-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.