Fix most strong mode warnings.

lib/src/result/capture_transformer.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 '../result.dart';
 import 'capture_sink.dart';
 
-/// Use [Result.captureTransformer] instead.
+/// A stream transformer that captures a stream of events into [Result]s.
+///
+/// The result of the transformation is a stream of [Result] values and no
+/// error events. This is the transformer used by [captureStream].
 @Deprecated("Will be removed in async 2.0.0.")
 class CaptureStreamTransformer<T> implements StreamTransformer<T, Result<T>> {
   const CaptureStreamTransformer();
 
   Stream<Result<T>> bind(Stream<T> source) {
     return new Stream<Result<T>>.eventTransformed(source, _createSink);
   }
 
   static EventSink _createSink(EventSink<Result> sink) {
     return new CaptureSink(sink);
   }
 }