Allow [path.fromUri] to take a string as well as a URI.

sdk/lib/_internal/pub/lib/src/barback/pub_package_provider.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 pub.pub_package_provider;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
 import 'package:path/path.dart' as path;
 
 import '../package_graph.dart';
 import '../io.dart';
 
 /// An implementation of barback's [PackageProvider] interface so that barback
 /// can find assets within pub packages.
 class PubPackageProvider implements PackageProvider {
   final PackageGraph _graph;
   final List<String> packages;
 
   PubPackageProvider(PackageGraph graph)
       : _graph = graph,
         packages = new List.from(graph.packages.keys)..add(r"$pub");
 
   Future<Asset> getAsset(AssetId id) {
     if (id.package != r'$pub') {
-      var nativePath = path.joinAll(path.url.split(id.path));
+      var nativePath = path.fromUri(id.path);
       var file = path.join(_graph.packages[id.package].dir, nativePath);
       return new Future.value(new Asset.fromPath(id, file));
     }
 
     // "$pub" is a psuedo-package that allows pub's transformer-loading
     // infrastructure to share code with pub proper.
     var components = path.url.split(id.path);
     assert(components.isNotEmpty);
     assert(components.first == 'lib');
     components[0] = 'dart';
     var file = assetPath(path.joinAll(components));
     return new Future.value(new Asset.fromPath(id, file));
   }
 }