Fix Dart 1.17 strong-mode warnings.

lib/src/typed/future.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';
 
 class TypeSafeFuture<T> implements Future<T> {
   final Future _future;
 
   TypeSafeFuture(this._future);
 
   Stream<T> asStream() => _future.then((value) => value as T).asStream();
 
-  Future catchError(Function onError, {bool test(Object error)}) =>
-      _future.catchError(onError, test: test);
+  Future<T> catchError(Function onError, {bool test(Object error)}) async =>
+      new TypeSafeFuture<T>(_future.catchError(onError, test: test));
 
   Future/*<S>*/ then/*<S>*/(/*=S*/ onValue(T value), {Function onError}) =>
       _future.then((value) => onValue(value as T), onError: onError);
 
   Future<T> whenComplete(action()) =>
       new TypeSafeFuture<T>(_future.whenComplete(action));
 
   Future<T> timeout(Duration timeLimit, {onTimeout()}) =>
       new TypeSafeFuture<T>(_future.timeout(timeLimit, onTimeout: onTimeout));
 }