StreamConsumer has an addStream and a close functions.

sdk/lib/io/file_impl.dart

 // Copyright (c) 2013, 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.
 
 part of dart.io;
 
 // Read the file in blocks of size 64k.
 const int _BLOCK_SIZE = 64 * 1024;
 
 
@@ skipping 133 matching lines @@
     } else {
       _resume();
     }
   }
 }
 
 class _FileStreamConsumer extends StreamConsumer<List<int>, File> {
   File _file;
   Future<RandomAccessFile> _openFuture;
   StreamSubscription _subscription;
+  
 
   _FileStreamConsumer(File this._file, FileMode mode) {
     _openFuture = _file.open(mode: mode);
   }
 
   _FileStreamConsumer.fromStdio(int fd) {
     assert(1 <= fd && fd <= 2);
     _openFuture = new Future.immediate(_File._openStdioSync(fd));
   }
 
   Future<File> consume(Stream<List<int>> stream) {
+    return addStream(stream).then((_) => close());
+  }
+
+  Future<File> addStream(Stream<List<int>> stream) {
     Completer<File> completer = new Completer<File>();
     _openFuture
       .then((openedFile) {
         _subscription = stream.listen(
           (d) {
             _subscription.pause();
             openedFile.writeList(d, 0, d.length)
               .then((_) => _subscription.resume())
               .catchError((e) {
                 openedFile.close();
                 completer.completeError(e);
               });
           },
           onDone: () {
-            // Wait for the file to close (and therefore flush) before
-            // completing the future.
-            openedFile.close()
-              .then((_) {
-                completer.complete(_file);
-              })
-              .catchError((e) {
-                completer.completeError(e);
-              });
+            completer.complete(_file);
           },
           onError: (e) {
             openedFile.close();
             completer.completeError(e);
           },
           unsubscribeOnError: true);
       })
       .catchError((e) {
         completer.completeError(e);
       });
     return completer.future;
   }
+
+  Future<File> close() {
+    return _openFuture.then((openedFile) => openedFile.close());
+  }
 }
 
 
 const int _EXISTS_REQUEST = 0;
 const int _CREATE_REQUEST = 1;
 const int _DELETE_REQUEST = 2;
 const int _OPEN_REQUEST = 3;
 const int _FULL_PATH_REQUEST = 4;
 const int _DIRECTORY_REQUEST = 5;
 const int _CLOSE_REQUEST = 6;
@@ skipping 840 matching lines @@
           new FileIOException("File closed '$_path'"));
     });
     return completer.future;
   }
 
   final String _path;
   int _id;
 
   SendPort _fileService;
 }