Don't load transformers that aren't going to be used for an executable.

sdk/lib/_internal/pub/test/dependency_computer/cycle_test.dart

 // Copyright (c) 2014, 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_tests;
 
 import '../descriptor.dart' as d;
 import '../test_pub.dart';
 import 'utils.dart';
 
@@ skipping 190 matching lines @@
       d.dir('lib', [
         d.file("myapp.dart", transformer(['foo.dart'])),
         d.file("foo.dart", "import 'bar.dart';"),
         d.file("bar.dart", "import 'myapp.dart';"),
       ])
     ]).create();
 
     expectDependencies({'myapp': []});
   });
 }