Introduce class Sink<T>.

sdk/lib/core/sink.dart

Index: sdk/lib/core/sink.dart
diff --git a/sdk/lib/core/sink.dart b/sdk/lib/core/sink.dart
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+// Copyright (c) 2014, 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.core;
+
+/**
+ * A generic destination for data.
+ *
+ * Multiple data values can be put into a sink, and when no more data is
+ * available, the sink should be closed.
+ *
+ * This is a generic interface that other data receivers can implement.
+ */
+abstract class Sink<T> {
+  /**
+   * Put the data into the sink.
+   *
+   * Must not be called after a call to [close].
+   */
+  void add(T data);
+
+  /**
+   * Tell the sink that no further data will be added.
+   *
+   * Calling this method more than once is allowed, but does nothing.
+   *
+   * The [add] method must not be called after this method.
+   */
+  void close();
+}