Move pub into sdk/lib/_internal.

sdk/lib/_internal/pub/lib/src/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 'dart:async';
 
 import 'package:pathos/path.dart' as path;
 
@@ skipping 202 matching lines @@
   /// Returns the [Package]s that have been installed in the system cache.
   List<Package> getCachedPackages() {
     if (shouldCache) {
       throw new UnimplementedError("Source $name must implement this.");
     }
   }
 
   /// Returns the source's name.
   String toString() => name;
 }