remove docgen source and targets from build

pkg/docgen/test/multi_library_code/lib/root_lib.dart

Index: pkg/docgen/test/multi_library_code/lib/root_lib.dart
diff --git a/pkg/docgen/test/multi_library_code/lib/root_lib.dart b/pkg/docgen/test/multi_library_code/lib/root_lib.dart
deleted file mode 100644
index 74423abfeba22b68b7561d308758df0fdd7d9e60..0000000000000000000000000000000000000000
--- a/pkg/docgen/test/multi_library_code/lib/root_lib.dart
+++ /dev/null
@@ -1,13 +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 root_lib;
-
-export 'test_lib.dart';
-export 'test_lib_foo.dart';
-export 'test_lib_bar.dart';
-
-class RootClass {
-  RootClass(int a, int b);
-}