pkg/async: fix spelling in doc comments

lib/result.dart

 // Copyright (c) 2013, 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.
 
 /**
  * Capture asynchronous results into synchronous values, and release them again.
  *
  * Capturing a result (either a returned value or a thrown error)
  * means converting it into a [Result] -
  * either a [ValueResult] or an [ErrorResult].
@@ skipping 79 matching lines @@
    * Shorthand for transforming the stream using [CaptureStreamTransformer].
    */
   static Stream<Result> captureStream(Stream source) {
     return source.transform(const CaptureStreamTransformer());
   }
 
   /**
    * Release a stream of [result] values into a stream of the results.
    *
    * `Result` values of the source stream become value or error events in
-   * the retuned stream as appropriate.
+   * the returned stream as appropriate.
    * Errors from the source stream become errors in the returned stream.
    *
    * Shorthand for transforming the stream using [ReleaseStreamTransformer].
    */
   static Stream releaseStream(Stream<Result> source) {
     return source.transform(const ReleaseStreamTransformer());
   }
 
   /**
    * Converts a result of a result to a single result.
@@ skipping 171 matching lines @@
     }
   }
   void addError(Object error, [StackTrace stackTrace]) {
     // Errors may be added by intermediate processing, even if it is never
     // added by CaptureSink.
     _sink.addError(error, stackTrace);
   }
 
   void close() { _sink.close(); }
 }