fix #25944, improve Future.then inference

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 415 matching lines @@
    * the future returned by `then` will be completed with
    * the same result as the future returned by the callback.
    *
    * If [onError] is not given, and this future completes with an error,
    * the error is forwarded directly to the returned future.
    *
    * In most cases, it is more readable to use [catchError] separately, possibly
    * with a `test` parameter, instead of handling both value and error in a
    * single [then] call.
    */
-  Future/*<S>*/ then/*<S>*/(/*=S*/ onValue(T value), { Function onError });
+  Future/*<S>*/ then/*<S>*/(onValue(T value), { Function onError });
 
   /**
    * Handles errors emitted by this [Future].
    *
    * This is the asynchronous equivalent of a "catch" block.
    *
    * Returns a new [Future] that will be completed with either the result of
    * this future or the result of calling the `onError` callback.
    *
    * If this future completes with a value,
@@ skipping 309 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();