pkg/docgen: moved model classes into minilibs

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";
+}