StreamConsumer has an addStream and a close functions.

tests/lib/async/slow_consumer_test.dart

 // 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.
 
 // VMOptions=--old_gen_heap_size=64
 
 library slow_consumer_test;
 
 import 'dart:async';
 import 'dart:isolate';
 
 const int KB = 1024;
 const int MB = KB * KB;
 const int GB = KB * KB * KB;
 
 class SlowConsumer extends StreamConsumer {
   var current = new Future.immediate(0);
   final int bytesPerSecond;
+  int finalCount;
 
   SlowConsumer(int this.bytesPerSecond);
 
   Future consume(Stream stream) {
+    return addStream(stream).then((_) => close());
+  }
+
+  Future addStream(Stream stream) {
     bool done = false;
     Completer completer = new Completer();
     var subscription;
     subscription = stream.listen(
       (List<int> data) {
         current = current
           .then((count) {
             // Simulated amount of time it takes to handle the data.
             int ms = data.length * 1000 ~/ bytesPerSecond;
             Duration duration = new Duration(milliseconds: ms);
             if (!done) subscription.pause();
             return new Future.delayed(duration, () {
               if (!done) subscription.resume();
               // Make sure we use data here to keep tracking it.
               return count + data.length;
             });
           });
         },
       onDone: () {
         done = true;
-        current.then((count) { completer.complete(count); });
+        current.then((count) {
+          finalCount = count;
+          completer.complete(count);
+        });
       });
     return completer.future;
   }
+
+  Future close() {
+    return new Future.immediate(finalCount);
+  }
 }
 
 class DataProvider {
   final int chunkSize;
   final int bytesPerSecond;
   int sentCount = 0;
   int targetCount;
   StreamController controller;
   Timer pendingSend;
 
@@ skipping 45 matching lines @@
   // file). If the consumer doesn't pause the data-provider it will run out of
   // heap-space.
 
   new DataProvider(800 * MB, 100 * MB, 1 * MB).stream
     .pipe(new SlowConsumer(200 * MB))
     .then((count) {
       port.close();
       Expect.equals(100 * MB, count);
     });
 }