remove docgen source and targets from build

pkg/docgen/lib/src/models/mirror_based.dart

-// Copyright (c) 2014, 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 docgen.models.mirror_based;
-
-import '../exports/source_mirrors.dart';
-
-/// Docgen representation of an item to be documented, that wraps around a
-/// dart2js mirror.
-abstract class MirrorBased<TMirror extends DeclarationMirror> {
-  /// The original dart2js mirror around which this object wraps.
-  TMirror get mirror;
-
-  /// Return an informative [Object.toString] for debugging.
-  String toString() => "${super.toString()} - $mirror";
-}