Add sinks to provide synchronous access to events.

lib/src/accumulator_sink.dart

Index: lib/src/accumulator_sink.dart
diff --git a/lib/src/accumulator_sink.dart b/lib/src/accumulator_sink.dart
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+++ b/lib/src/accumulator_sink.dart
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+// Copyright (c) 2016, 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.
+
+import 'dart:collection';
+
+/// A sink that provides access to all the [events] that have been passed to it.
+///
+/// See also [ChunkedConversionSink.withCallback].
+class AccumulatorSink<T> implements Sink<T> {
+  /// An unmodifiable list of events passed to this sink so far.
+  List<T> get events => new UnmodifiableListView(_events);
+  final _events = <T>[];
+
+  /// Whether [close] has been called.
+  bool get isClosed => _isClosed;
+  var _isClosed = false;
+
+  /// Removes all events from [events].
+  ///
+  /// This can be used to avoid double-processing events.
+  void clear() {
+    _events.clear();
+  }
+
+  void add(T event) {
+    if (_isClosed) {
+      throw new StateError("Can't add to a closed sink.");
+    }
+
+    _events.add(event);
+  }
+
+  void close() {
+    _isClosed = true;
+  }
+}