First pass at build dependency graph for barback.

pkg/barback/lib/src/errors.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 barback.errors;
+
+import 'dart:async';
+import 'dart:io';
+
+import '../barback.dart';
+
+/// Error thrown when an asset with [id] cannot be found.
+class AssetNotFoundException implements Exception {
+  final AssetId id;
+
+  AssetNotFoundException(this.id);
+
+  String toString() => "Could not find asset $id.";
+}
+
+/// Error thrown when two transformers both output an asset with [id].
+class AssetCollisionException implements Exception {
+  final AssetId id;
+
+  AssetCollisionException(this.id);
+
+  String toString() => "Got collision on asset $id.";
+}
+
+/// Error thrown when a transformer requests an input [id] which cannot be
+/// found.
+class MissingInputException implements Exception {
+  final AssetId id;
+
+  MissingInputException(this.id);
+
+  String toString() => "Missing input $id.";
+}