Allow transformers to exclude and include assets.

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

Index: sdk/lib/_internal/pub/lib/src/barback/excluding_transformer.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/excluding_transformer.dart b/sdk/lib/_internal/pub/lib/src/barback/excluding_transformer.dart
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index 0000000000000000000000000000000000000000..03b485b6408165f274bfc9eeaa2bae0cfa8beff2
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+++ b/sdk/lib/_internal/pub/lib/src/barback/excluding_transformer.dart
@@ -0,0 +1,58 @@
+// 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.excluding_transformer;
+
+import 'dart:async';
+
+import 'package:barback/barback.dart';
+
+/// Decorates an inner [Transformer] and handles including and excluding
+/// primary inputs.
+class ExcludingTransformer extends Transformer {
+  /// If [includes] or [excludes] is non-null, wraps [inner] in an
+  /// [ExcludingTransformer] that handles those.
+  ///
+  /// Otherwise, just returns [inner] unmodified.
+  static Transformer wrap(Transformer inner, Set<String> includes,
+      Set<String> excludes) {
+    if (includes == null && excludes == null) return inner;
+
+    return new ExcludingTransformer._(inner, includes, excludes);
+  }
+
+  final Transformer _inner;
+
+  /// The set of asset paths which should be included.
+  ///
+  /// If `null`, all non-excluded assets are allowed. Otherwise, only included
+  /// assets are allowed.
+  final Set<String> _includes;
+
+  /// The set of assets which should be excluded.
+  ///
+  /// Exclusions are applied after inclusions.
+  final Set<String> _excludes;
+
+  ExcludingTransformer._(this._inner, this._includes, this._excludes);
+
+  Future<bool> isPrimary(Asset asset) {
+    // TODO(rnystrom): Support globs in addition to paths. See #17093.
+    if (_includes != null) {
+      // If there are any includes, it must match one of them.
+      if (!_includes.contains(asset.id.path)) return new Future.value(false);
+    }
+
+    // It must not be excluded.
+    if (_excludes != null && _excludes.contains(asset.id.path)) {
+      return new Future.value(false);
+    }
+
+    return _inner.isPrimary(asset);
+  }
+
+  Future apply(Transform transform) => _inner.apply(transform);
+
+  String toString() => _inner.toString();
+}