Use first-class types to clean up pub's transformer loading a bit.

sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart

Index: sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart b/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
index 7afe4d18fab2c9089d81827912a12edae087a063..be6ef7cf5a6e8a4c6dec38bdd230dc2c92f09364 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
@@ -46,11 +46,7 @@ void main() {
 /// them (with [configuration] if it's non-null), and return them.
 Iterable<Transformer> initialize(Uri uri, Map configuration) {
   var mirrors = currentMirrorSystem();
-  // TODO(nweiz): look this up by name once issue 5897 is fixed.
-  var transformerUri = Uri.parse(
-      'http://<<HOST_AND_PORT>>/packages/barback/src/transformer.dart');
-  var transformerClass = mirrors.libraries[transformerUri]
-      .classes[const Symbol('Transformer')];
+  var transformerClass = reflectClass(Transformer);
 
   // TODO(nweiz): if no valid transformers are found, throw an error message
   // describing candidates and why they were rejected.