Change Hash subclasses to be Converters.

lib/src/digest_sink.dart

Index: lib/src/digest_sink.dart
diff --git a/lib/src/digest_sink.dart b/lib/src/digest_sink.dart
new file mode 100644
index 0000000000000000000000000000000000000000..bb5c85f496d670a60d95a5fcfccbd16d87c326a6
--- /dev/null
+++ b/lib/src/digest_sink.dart
@@ -0,0 +1,29 @@
+// Copyright (c) 2015, 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.
+
+library crypto.digest_sink;
+
+import 'digest.dart';
+
+/// A sink used to get a digest value out of [Hash.startChunkedConversion].
+class DigestSink extends Sink<Digest> {
+  /// The value added to the sink, if any.
+  Digest get value {
+    assert(_value != null);
+    return _value;
+  }
+  Digest _value;
+
+  /// Adds [value] to the sink.
+  ///
+  /// Unlike most sinks, this may only be called once.
+  void add(Digest value) {
+    assert(_value == null);
+    _value = value;
+  }
+
+  void close() {
+    assert(_value != null);
+  }
+}