remove docgen source and targets from build

pkg/docgen/test/multi_library_code/lib/test_lib_bar.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 test_lib.bar;
-
-import 'test_lib.dart';
-import 'test_lib_foo.dart';
-
-/*
- * Normal comment for class C.
- */
-class C {
-}
-
-/// Processes an [input] of type [C] instance for testing.
-///
-/// To eliminate import warnings for [A] and to test typedefs.
-///
-/// It's important that the [List<A>] for param [listOfA] is not empty.
-A testMethod(C input, List<A> listOfA, B aBee) {
-  throw 'noop';
-}
-
-/// Processes an [input] of type [C] instance for testing.
-///
-/// To eliminate import warnings for [A] and to test typedefs.
-///
-/// It's important that the [List<A>] for param [listOfA] is not empty.
-typedef A testTypedef(C other, List<A> listOfA, B aBee);