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

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

 library pub_tests;
 import '../descriptor.dart' as d;
 import '../test_pub.dart';
 import 'utils.dart';
 void main() {
   initConfig();
   integration(
       "allows a package dependency cycle that's unrelated to " "transformers",
       () {
     d.dir(appPath, [d.pubspec({
@@ skipping 211 matching lines @@
               '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': []
     });
   });
 }