First pass at build dependency graph for barback.

pkg/barback/lib/src/transformer.dart

Index: pkg/barback/lib/src/transformer.dart
diff --git a/pkg/barback/lib/src/transformer.dart b/pkg/barback/lib/src/transformer.dart
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+// 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 barback.transformer;
+
+import 'dart:async';
+import 'dart:io';
+
+import 'asset_id.dart';
+import 'transform.dart';
+
+/// A [Transformer] represents a processor that takes in one or more input
+/// assets and uses them to generate one or more output assets.
+///
+/// Dart2js, a SASS->CSS processor, a CSS spriter, and a tool to concatenate
+/// files are all examples of transformers. To define your own transformation
+/// step, extend (or implement) this class.
+abstract class Transformer {
+  /// Returns `true` if [input] can be a primary input for this transformer.
+  ///
+  /// While a transformer can read from multiple input files, one must be the
+  /// "primary" input. This asset determines whether the transformation should
+  /// be run at all. If the primary input is removed, the transformer will no
+  /// longer be run.
+  ///
+  /// A concrete example is dart2js. When you run dart2js, it will traverse
+  /// all of the imports in your Dart source files and use the contents of all
+  /// of those to generate the final JS. However you still run dart2js "on" a
+  /// single file: the entrypoint Dart file that has your `main()` method.
+  /// This entrypoint file would be the primary input.
+  Future<bool> isPrimary(AssetId input);
+
+  /// Run this transformer on on the primary input specified by [transform].
+  ///
+  /// The [transform] is used by the [Transformer] for two purposes (in
+  /// addition to accessing the primary input). It can call `getInput()` to
+  /// request additional input assets. It also calls `addOutput()` to provide
+  /// generated assets back to the system. Either can be called multiple times,
+  /// in any order.
+  ///
+  /// In other words, a Transformer's job is to find all inputs for a
+  /// transform, starting at the primary input, then generate all output assets
+  /// and yield them back to the transform.
+  Future apply(Transform transform);
+
+  String toString() => runtimeType.toString().replaceAll("Transformer", "");
+}