Add stack chain support to pub.

sdk/lib/_internal/pub/test/version_solver_test.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.
 
 library pub_upgrade_test;
 
 import 'dart:async';
 
 import 'package:unittest/unittest.dart';
 
 import '../lib/src/lock_file.dart';
 import '../lib/src/log.dart' as log;
 import '../lib/src/package.dart';
 import '../lib/src/pubspec.dart';
 import '../lib/src/sdk.dart' as sdk;
 import '../lib/src/source.dart';
 import '../lib/src/system_cache.dart';
+import '../lib/src/utils.dart';
 import '../lib/src/version.dart';
 import '../lib/src/solver/version_solver.dart';
 import 'test_pub.dart';
 
 MockSource source1;
 MockSource source2;
 
 main() {
   initConfig();
 
@@ skipping 1235 matching lines @@
   final String name;
   bool get shouldCache => true;
 
   MockSource(this.name);
 
   Future<String> systemCacheDirectory(PackageId id) {
     return new Future.value('${id.name}-${id.version}');
   }
 
   Future<List<Version>> getVersions(String name, String description) {
-    return new Future.sync(() {
+    return syncFuture(() {
       // Make sure the solver doesn't request the same thing twice.
       if (_requestedVersions.contains(description)) {
         throw new Exception('Version list for $description was already '
             'requested.');
       }
 
       _requestedVersions.add(description);
 
       if (!_packages.containsKey(description)){
         throw new Exception('MockSource does not have a package matching '
             '"$description".');
       }
 
       return _packages[description].keys.toList();
     });
   }
 
   Future<Pubspec> describe(PackageId id) {
-    return new Future.sync(() {
+    return syncFuture(() {
       // Make sure the solver doesn't request the same thing twice.
       if (_requestedPubspecs.containsKey(id.description) &&
           _requestedPubspecs[id.description].contains(id.version)) {
         throw new Exception('Pubspec for $id was already requested.');
       }
 
       _requestedPubspecs.putIfAbsent(id.description, () => new Set<Version>());
       _requestedPubspecs[id.description].add(id.version);
 
       return _packages[id.description][id.version].pubspec;
@@ skipping 94 matching lines @@
   }
 
   var source = "mock1";
   if (match[7] != null) {
     source = match[7];
     if (source == "root") source = null;
   }
 
   return new PackageId(name, source, parsedVersion, description);
 }