Get rid of RootSource.

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 'dart:async';
-import 'package.dart';
-import 'pubspec.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<Pubspec> describe(PackageId id) {
-    return new Future<Pubspec>.immediate(package.pubspec);
-  }
-
-  Future<bool> install(PackageId id, String destPath) {
-    throw new UnsupportedError("Can't install from a root source.");
-  }
-}