Support loading transformer plugins from 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_update_test;
 
 import 'dart:async';
 
 import 'package:unittest/unittest.dart';
 
@@ skipping 1154 matching lines @@
         devDependencies.add(dep);
       } else {
         dependencies.add(dep);
       }
     });
   });
 
   var name = description.replaceFirst(new RegExp(r"-[^-]+$"), "");
   var pubspec = new Pubspec(
       name, new Version.parse(version), dependencies, devDependencies,
-      new PubspecEnvironment(sdkConstraint));
+      new PubspecEnvironment(sdkConstraint), []);
   return new Package.inMemory(pubspec);
 }
 
 void parseSource(String description,
     callback(bool isDev, String name, String source)) {
   var isDev = false;
 
   if (description.startsWith("(dev) ")) {
     description = description.substring("(dev) ".length);
     isDev = true;
   }
 
   var name = description;
   var source = "mock1";
   var match = new RegExp(r"(.*) from (.*)").firstMatch(description);
   if (match != null) {
     name = match[1];
     source = match[2];
     if (source == "root") source = null;
   }
 
   callback(isDev, name, source);
 }