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

sdk/lib/_internal/pub/test/dependency_computer/conservative_dependencies_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 'package:scheduled_test/scheduled_test.dart';
 
 import '../descriptor.dart' as d;
 import '../test_pub.dart';
@@ skipping 446 matching lines @@
         "name": "baz",
         "version": "1.0.0",
         "transformers": ["baz"]
       }),
       d.dir("lib", [d.file("baz.dart", transformer())])
     ]).create();
 
     expectDependencies({'pkg': ['foo'], 'foo': [], 'bar': [], 'baz': []});
   });
 }