Expose aggregate transformers in barback.

pkg/barback/test/transformer/aggregate_many_to_one.dart

+// 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 barback.test.transformer.aggregate_many_to_one;
+
+import 'dart:async';
+
+import 'package:barback/barback.dart';
+import 'package:path/path.dart' as path;
+
+import 'mock_aggregate.dart';
+
+/// An [AggregateTransformer] that takes all assets in each directory with a
+/// given extension and concatenates them in alphabetic order by filename.

[Bob Nystrom, 2014/05/27 17:20:18]

This is ambiguous (and a bit hard to read). How about:

"... applies to all assets with a given extension. For each directory containing
any of these assets, it produces an output file that contains the concatenation
of all matched assets in that directory."

[nweiz, 2014/05/27 21:03:10]

Done.

+class AggregateManyToOneTransformer extends MockAggregateTransformer {
+  /// The extension of assets to combine.
+  final String extension;
+
+  /// The basename of the output asset.

[Bob Nystrom, 2014/05/27 17:20:18]

Mention that the output includes the directory name too.

[nweiz, 2014/05/27 21:03:10]

Done.

+  final String output;
+
+  AggregateManyToOneTransformer(this.extension, this.output);
+
+  String doClassifyPrimary(AssetId id) {
+    if (id.extension != ".$extension") return null;
+    return path.url.dirname(id.path);
+  }
+
+  Future doApply(AggregateTransform transform) {
+    return getPrimaryInputs(transform).toList().then((assets) {
+      assets.sort((asset1, asset2) => asset1.id.path.compareTo(asset2.id.path));
+      return Future.wait(assets.map((asset) => asset.readAsString()));
+    }).then((contents) {
+      var id = new AssetId(transform.package,
+          path.url.join(transform.key, output));
+      transform.addOutput(new Asset.fromString(id, contents.join('\n')));
+    });
+  }
+
+  String toString() => "aggregate $extension->$output";
+}