Free the C memory for ArchiveInputStream when the stream is garbage collected.

utils/archive/reader.c

 // 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 "messaging.h"
 #include "reader.h"
 
 void archiveReadNew(Dart_Port p) {
   struct archive* a = archive_read_new();
   DART_INT64(result, (intptr_t) a)
@@ skipping 323 matching lines @@
 }
 
 // TODO(nweiz): wrap archive_read_into_fd when issue 4160 is fixed
 // TODO(nweiz): wrap archive_read_extract and friends
 
 void archiveReadClose(Dart_Port p, struct archive* a) {
   checkResult(p, a, archive_read_close(a));
 }
 
 void archiveReadFree(Dart_Port p, struct archive* a) {
-  // TODO(nweiz): Should we attach a finalizer to the Dart object that calls
-  // this automatically?
   checkResult(p, a, archive_read_free(a));
 }