Add support to the version solver for sources that only have a single version per package.

utils/pub/root_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('root_source');
 
 #import('package.dart');
 #import('source.dart');
 
 /**
  * A source used only for the root package when doing version resolution. It
  * contains only the root package and is unable to install packages.
  *
  * This source cannot be referenced from a pubspec.
  */
 class RootSource extends Source {
   final String name = "root";
   final bool shouldCache = false;
   final Package package;
 
   RootSource(this.package);
 
-  Future<List<Version>> getVersions(description) {
-    return Future<List<Version>>.immediate([package.version]);
-  }
-
   Future<Pubspec> describe(PackageId id) {
     return Future<Pubspec>.immediate(package.pubspec);
   }
 
   Future<bool> install(PackageId id, String destPath) {
     throw new UnsupportedOperationException(
         "Can't install from a root source.");
   }
 }