Move all main methods to tool/.

pkg/front_end/tool/fasta/generate_dart_libraries.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.
 
 import 'package:sdk_library_metadata/libraries.dart'
     show LibraryInfo, libraries;
 
 void main(_) {
   libraries.forEach((String name, LibraryInfo info) {
     if (info.isVmLibrary) {
       print('"$name": sdk.resolve("lib/${info.path}"),');
     }
   });
 }