Refactor Future constructors.

pkg/scheduled_test/lib/src/descriptor/pattern_descriptor.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.
 
 library descriptor.pattern;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:pathos/path.dart' as path;
@@ skipping 92 matching lines @@
     if (pattern is String) return "'$pattern'";
     if (pattern is! RegExp) return '$pattern';
 
     var regExp = pattern as RegExp;
     var flags = new StringBuffer();
     if (!regExp.isCaseSensitive) flags.write('i');
     if (regExp.isMultiLine) flags.write('m');
     return '/${regExp.pattern}/$flags';
   }
 
-  Future create([String parent]) => new Future.immediateError(
+  Future create([String parent]) => new Future.error(
       new UnsupportedError("Pattern descriptors don't support create()."));
 
   Stream<List<int>> load(String pathToLoad) => errorStream(
       new UnsupportedError("Pattern descriptors don't support load()."));
 
   Stream<List<int>> read() => errorStream(new UnsupportedError("Pattern "
       "descriptors don't support read()."));
 }