Create a "PackageRef" class that defines a versionless package reference.

sdk/lib/_internal/pub/lib/src/path_source.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 path_source;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:pathos/path.dart' as path;
 
 import 'log.dart' as log;
 
 import 'io.dart';
 import 'package.dart';
 import 'pubspec.dart';
 import 'version.dart';
 import 'source.dart';
 import 'utils.dart';
 
-// TODO(rnystrom): Support relative paths. (See comment in _validatePath().)
 /// A package [Source] that installs packages from a given local file path.
 class PathSource extends Source {
   final name = 'path';
   final shouldCache = false;
 
   Future<Pubspec> describe(PackageId id) {
     return new Future.sync(() {
       _validatePath(id.name, id.description);
       return new Pubspec.load(id.name, id.description["path"],
           systemCache.sources);
@@ skipping 87 matching lines @@
 
     if (fileExists(dir)) {
       throw new FormatException(
           "Path dependency for package '$name' must refer to a "
           "directory, not a file. Was '$dir'.");
     }
 
     throw new FormatException("Could not find package '$name' at '$dir'.");
   }
 }