Add sinks to provide synchronous access to events.

test/accumulator_sink_test.dart

+// 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:async';
+
+import 'package:convert/convert.dart';
+import 'package:test/test.dart';
+
+void main() {
+  var sink;
+  setUp(() {
+    sink = new AccumulatorSink<int>();
+  });
+
+  test("provides access to events as they're added", () {
+    expect(sink.events, isEmpty);
+
+    sink.add(1);
+    expect(sink.events, equals([1]));
+
+    sink.add(2);
+    expect(sink.events, equals([1, 2]));
+
+    sink.add(3);
+    expect(sink.events, equals([1, 2, 3]));
+  });
+
+  test("indicates whether the sink is closed", () {
+    expect(sink.isClosed, isFalse);
+    sink.close();
+    expect(sink.isClosed, isTrue);
+  });
+
+  test("doesn't allow add() to be called after close()", () {
+    sink.close();
+    expect(() => sink.add(1), throwsStateError);
+  });
+}