Reorganize some pub libraries.

sdk/lib/_internal/pub/lib/src/error_group.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 error_group;
+library pub.error_group;
 
 import 'dart:async';
 
 import 'utils.dart';
 
 /// An [ErrorGroup] entangles the errors of multiple [Future]s and [Stream]s
 /// with one another. This allows APIs to expose multiple [Future]s and
 /// [Stream]s that have identical error conditions without forcing API consumers
 /// to attach error handling to objects they don't care about.
 ///
@@ skipping 252 matching lines @@
   void _signalError(var e) {
     if (_isDone) return;
     _subscription.cancel();
     // Call these asynchronously to work around issue 7913.
     new Future.value().then((_) {
       _controller.addError(e);
       _controller.close();
     });
   }
 }