Add new features to package:async.

lib/src/delegating_stream_subscription.dart

Index: lib/src/delegating_stream_subscription.dart
diff --git a/lib/src/delegating_stream_subscription.dart b/lib/src/delegating_stream_subscription.dart
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+// 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 async.delegating_stream_subscription;
+
+import 'dart:async';
+
+/// Simple delegating wrapper around a [StreamSubscription].
+///
+/// Subclasses can override individual methods.
+class DelegatingStreamSubscription<T> implements StreamSubscription<T> {
+  final StreamSubscription _source;
+
+  /// Create delegating subscription forwarding calls to [sourceSubscription].
+  DelegatingStreamSubscription(StreamSubscription sourceSubscription)
+      : _source = sourceSubscription;
+
+  void onData(void handleData(T data)) {
+    _source.onData(handleData);
+  }
+
+  void onError(Function handleError) {
+    _source.onError(handleError);
+  }
+
+  void onDone(void handleDone()) {
+    _source.onDone(handleDone);
+  }
+
+  void pause([Future resumeFuture]) {
+    _source.pause(resumeFuture);
+  }
+
+  void resume() {
+    _source.resume();
+  }
+
+  Future cancel() => _source.cancel();
+
+  Future asFuture([futureValue]) => _source.asFuture(futureValue);
+
+  bool get isPaused => _source.isPaused;
+}