remove docgen source and targets from build

pkg/docgen/lib/src/exports/mirrors_util.dart

Index: pkg/docgen/lib/src/exports/mirrors_util.dart
diff --git a/pkg/docgen/lib/src/exports/mirrors_util.dart b/pkg/docgen/lib/src/exports/mirrors_util.dart
deleted file mode 100644
index f02b55e560a17150cf1935c79cfd8aa259c9dbc0..0000000000000000000000000000000000000000
--- a/pkg/docgen/lib/src/exports/mirrors_util.dart
+++ /dev/null
@@ -1,7 +0,0 @@
-// 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.exports.mirrors_util;
-
-export 'package:compiler/src/mirrors/mirrors_util.dart';