Don't load dependencies' useless transformers.

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

 // Copyright (c) 2013, 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.barback.transformer_config;
 
+import 'package:path/path.dart' as p;
 import 'package:source_maps/source_maps.dart';
 import 'package:yaml/yaml.dart';
 
 import 'transformer_id.dart';
 
 /// The configuration for a transformer.
 ///
 /// This corresponds to the transformers listed in a pubspec, which have both an
 /// [id] indicating the location of the transformer and configuration specific
 /// to that use of the transformer.
@@ skipping 29 matching lines @@
   ///
   /// This is processed after [includes]. If a transformer has both includes
   /// and excludes, then the set of included assets is determined and assets
   /// are excluded from that resulting set.
   final Set<String> excludes;
 
   /// Returns whether this config excludes certain asset ids from being
   /// processed.
   bool get hasExclusions => includes != null || excludes != null;
 
+  /// Returns whether this transformer might transform a file that's visible to
+  /// the package's dependers.
+  bool get canTransformPublicFiles {
+    if (includes == null) return true;
+    return includes.any((path) =>
+        p.url.isWithin('lib', path) || p.url.isWithin('bin', path));
+  }
+
   /// Parses [identifier] as a [TransformerId] with [configuration].
   ///
   /// [identifierSpan] is the source span for [identifier].
   factory TransformerConfig.parse(String identifier, Span identifierSpan,
         YamlMap configuration) =>
       new TransformerConfig(new TransformerId.parse(identifier, identifierSpan),
           configuration);
 
   factory TransformerConfig(TransformerId id, YamlMap configurationNode) {
     parseField(key) {
@@ skipping 61 matching lines @@
     // TODO(rnystrom): Support globs in addition to paths. See #17093.
     if (excludes != null) {
       // If there are any excludes, it must not match any of them.
       if (excludes.contains(pathWithinPackage)) return false;
     }
 
     // If there are any includes, it must match one of them.
     return includes == null || includes.contains(pathWithinPackage);
   }
 }