Tweak public APIs and use them in patch_sdk, dart2js, and kernel-service

pkg/front_end/lib/front_end.dart

+// Copyright (c) 2017, 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.
+
+/// The Dart Front End contains logic to build summaries and kernel programs
+/// from Dart sources. The APIs exposed here are designed for tools in the Dart
+/// ecosystem that need to load sources and convert them to these formats.
+library front_end.front_end;
+
+export 'compiler_options.dart';
+export 'compilation_error.dart';
+export 'kernel_generator.dart';
+export 'summary_generator.dart';
+export 'file_system.dart';