Normalize incoming AssetID paths.

pkg/barback/lib/src/asset_id.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.asset_id;
 
 import 'package:path/path.dart' as pathos;
 
-/// AssetIDs always use POSIX style paths regardless of the host platform.
-final _posix = new pathos.Builder(style: pathos.Style.posix);
-
 /// Identifies an asset within a package.
 class AssetId implements Comparable<AssetId> {
   /// The name of the package containing this asset.
   final String package;
 
   /// The path to the asset relative to the root directory of [package].
   ///
   /// Source (i.e. read from disk) and generated (i.e. the output of a
   /// [Transformer]) assets all have paths. Even intermediate assets that are
   /// generated and then consumed by later transformations will still have
   /// a path used to identify it.
   ///
   /// Asset paths always use forward slashes as path separators, regardless of
   /// the host platform.
   final String path;
 
   /// Gets the file extension of the asset, if it has one, including the ".".
   String get extension => pathos.extension(path);
 
   /// Creates a new AssetId at [path] within [package].
+  ///
+  /// The [path] will be normalized: any backslashes will be replaced with
+  /// forward slashes (regardless of host OS) and "." and ".." will be removed
+  /// where possible.
   AssetId(this.package, String path)
-      : path = _posix.normalize(path);
+      : path = _normalizePath(path);
 
   /// Parses an [AssetId] string of the form "package|path/to/asset.txt".
+  ///
+  /// The [path] will be normalized: any backslashes will be replaced with
+  /// forward slashes (regardless of host OS) and "." and ".." will be removed
+  /// where possible.
   factory AssetId.parse(String description) {
     var parts = description.split("|");
     if (parts.length != 2) {
       throw new FormatException('Could not parse "$description".');
     }
 
     if (parts[0].isEmpty) {
       throw new FormatException(
           'Cannot have empty package name in "$description".');
     }
@@ skipping 39 matching lines @@
   /// but with file extension [newExtension].
   AssetId changeExtension(String newExtension) =>
       new AssetId(package, pathos.withoutExtension(path) + newExtension);
 
   String toString() => "$package|$path";
 
   /// Serializes this [AssetId] to an object that can be sent across isolates
   /// and passed to [deserialize].
   serialize() => [package, path];
 }
+
+String _normalizePath(String path) {
+  if (pathos.isAbsolute(path)) {
+    throw new ArgumentError('Asset paths must be relative, but got "$path".');
+  }
+
+  // Normalize path separators so that they are always "/" in the AssetID.
+  path = path.replaceAll(r"\", "/");
+
+  // Collapse "." and "..".
+  return pathos.posix.normalize(path);
+}