Add a StreamSinkCompleter class.

test/utils.dart

Index: test/utils.dart
diff --git a/test/utils.dart b/test/utils.dart
index 0dc47fa1521120b567a89c10adefc3aa3a173928..445b9fcffc7b184c9c427e5f90a047abc147f5a2 100644
--- a/test/utils.dart
+++ b/test/utils.dart
@@ -6,6 +6,8 @@
 library async.test.util;
 
 import "dart:async";
+
+import "package:async/async.dart";
 import "package:test/test.dart";
 
 /// A zero-millisecond timer should wait until after all microtasks.
@@ -40,3 +42,46 @@ class CompleterStreamSink<T> implements StreamSink<T> {
   Future addStream(Stream<T> stream) async {}
   Future close() => completer.future;
 }
+
+/// A [StreamSink] that collects all events added to it as results.
+///
+/// This is used for testing code that interacts with sinks.
+class TestSink<T> implements StreamSink<T> {
+  /// The results corresponding to events that have been added to the sink.
+  final results = <Result<T>>[];
+
+  /// Whether [close] has been called.
+  bool get isClosed => _isClosed;
+  var _isClosed = false;
+
+  Future get done => _doneCompleter.future;
+  final _doneCompleter = new Completer();
+
+  final Function _onDone;
+
+  /// Creates a new sink.
+  ///
+  /// If [onDone] is passed, it's called when the user calls [close]. Its result
+  /// is piped to the [done] future.
+  TestSink({onDone()}) : _onDone = onDone ?? (() {});
+
+  void add(T event) {
+    results.add(new Result<T>.value(event));
+  }
+
+  void addError(error, [StackTrace stackTrace]) {
+    results.add(new Result<T>.error(error, stackTrace));
+  }
+
+  Future addStream(Stream<T> stream) {
+    var completer = new Completer.sync();
+    stream.listen(add, onError: addError, onDone: completer.complete);
+    return completer.future;
+  }
+
+  Future close() {
+    _isClosed = true;
+    _doneCompleter.complete(new Future.microtask(_onDone));
+    return done;
+  }
+}