remove docgen source and targets from build

pkg/docgen/test/multi_library_code/lib/dottedLibraryName.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 library.name.wiith.dots.init;
-
-/// This library is purely for testing purposes, to ensure
-/// that we have a library that has dots in its name and
-/// that it works properly
-///
-/// In addition, it's nice to have something which has paragraph
-/// breaks inside a triple-slash doc comment.
-///
-/// Like this.
-
-/// This is a top level field holding the number three;
-int topLevelInt = 3;
-
-/// This is a class with various ways of getting the number three.
-///
-/// It also has a comment with paragraph breaks.
-class SomeClass {
-  /// This is a method that returns the number three. See
-  /// [three]
-  someMethod() => 3;
-
-  /// This is a symbolic representation of the number three.
-  int three = 3;
-}
-
-/// This is another class.
-class AnotherClass {
-  /// This method returns [List<int>] containing the number
-  /// three three times. Compare with [SomeClass.someMethod].
-  List<int> anotherMethod() {
-    return const [3, 3, 3];
-  }
-}