Support serving from multiple directories using "pub serve".

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 30aefdee9abfdfb7f65a873e5b99536546e868d5..13567edcdbb7160869936af5ed02b0104fff68fc 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
@@ -366,8 +366,11 @@ Future<Set> loadTransformers(BuildEnvironment environment, TransformerId id) {
   return id.getAssetId(environment.barback).then((assetId) {
     var path = assetId.path.replaceFirst('lib/', '');
     // TODO(nweiz): load from a "package:" URI when issue 12474 is fixed.
-    var baseUrl = baseUrlForAddress(environment.server.address,
-                                    environment.server.port);
+
+    // We could load the transformers from any server, since they all serve the
+    // packages' library files. We choose the first one arbitrarily.
+    var baseUrl = baseUrlForAddress(environment.servers.first.address,
+                                    environment.servers.first.port);
     var uri = '$baseUrl/packages/${id.package}/$path';
     var code = 'import "$uri";\n' +
         _TRANSFORMER_ISOLATE.replaceAll('<<URL_BASE>>', baseUrl);