Fix typo in Completer.sync documentation.

sdk/lib/async/future.dart

 // Copyright (c) 2012, 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.async;
 
 /**
  * An object representing a delayed computation.
  *
  * A [Future] is used to represent a potential value, or error,
@@ skipping 607 matching lines @@
    * Completes the future synchronously.
    *
    * This constructor should be avoided unless the completion of the future is
    * known to be the final result of another asynchronous operation. If in doubt
    * use the default [Completer] constructor.
    *
    * Using an normal, asynchronous, completer will never give the wrong
    * behavior, but using a synchronous completer incorrectly can cause
    * otherwise correct programs to break.
    *
-   * An asynchronous completer is only intended for optimizing event
+   * A synchronous completer is only intended for optimizing event
    * propagation when one asynchronous event immediately triggers another.
    * It should not be used unless the calls to [complete] and [completeError]
    * are guaranteed to occur in places where it won't break `Future` invariants.
    *
    * Completing synchronously means that the completer's future will be
    * completed immediately when calling the [complete] or [completeError]
    * method on a synchronous completer, which also calls any callbacks
    * registered on that future.
    *
    * Completing synchronously must not break the rule that when you add a
@@ skipping 81 matching lines @@
   if (replacement != null) {
     error = _nonNullError(replacement.error);
     stackTrace = replacement.stackTrace;
   }
   result._completeError(error, stackTrace);
 }
 
 /** Helper function that converts `null` to a [NullThrownError]. */
 Object _nonNullError(Object error) =>
   (error != null) ? error : new NullThrownError();