Add support for additional libraries to FE API

pkg/front_end/lib/compiler_options.dart

 // Copyright (c) 2016, 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 front_end.compiler_options;
 
 import 'compilation_error.dart';
 import 'file_system.dart';
 import 'physical_file_system.dart';
 
@@ skipping 109 matching lines @@
   // TODO(sigmund): revisit the right layout for these options. We might want to
   // split them out into a separate bag of options or provide factories for
   // common combinations of these options.
 
   /// Patch files to apply on the core libraries for a specific target platform.
   ///
   /// Keys on this map are expected to be `dart:*` URIs. The values can be
   /// either absolute or relative URIs. Absolute URIs are read directly, while
   /// relative URIs are resolved from the [sdkRoot].
   Map<Uri, List<Uri>> targetPatches = {};
+
+  /// Additional core libraries to be loaded when building a program.
+  // TODO(sigmund): delete. Ideally building a program only needs what's
+  // reachable and we can use kernelForBuildUnit when creating a snapshot of the
+  // SDK itself.
+  List<Uri> additionalLibraries = [];
 }