Better error messages for dependency conflicts.

utils/pub/source.dart

 // Copyright (c) 2012, 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 source;
 
 import 'io.dart';
 import 'package.dart';
 import 'pubspec.dart';
 import 'system_cache.dart';
@@ skipping 146 matching lines @@
    * [fromLockFile] is true when the description comes from a [LockFile], to
    * allow the source to use lockfile-specific descriptions via [resolveId].
    */
   void validateDescription(description, {bool fromLockFile: false}) {}
 
   /**
    * Returns whether or not [description1] describes the same package as
    * [description2] for this source. This method should be light-weight. It
    * doesn't need to validate that either package exists.
    *
-   * By default, this assumes both descriptions are strings and compares them
-   * for equality.
+   * By default, just uses regular equality.
    */
   bool descriptionsEqual(description1, description2) =>
     description1 == description2;
 
   /**
    * For some sources, [PackageId]s can point to different chunks of code at
    * different times. This takes such an [id] and returns a future that
    * completes to a [PackageId] that will uniquely specify a single chunk of
    * code forever.
    *
@@ skipping 10 matching lines @@
    * to JSON and YAML. It must also be equal to [id] according to
    * [descriptionsEqual].
    *
    * By default, this just returns [id].
    */
   Future<PackageId> resolveId(PackageId id) => new Future.immediate(id);
 
   /// Returns the source's name.
   String toString() => name;
 }