Move all main methods to tool/.

pkg/front_end/lib/src/fasta/scanner/main.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.md file.
-
-library fasta.scanner.main;
-
-import 'io.dart' show readBytesFromFileSync;
-
-import '../scanner.dart' show scan;
-
-scanAll(Map<Uri, List<int>> files) {
-  Stopwatch sw = new Stopwatch()..start();
-  int byteCount = 0;
-  files.forEach((Uri uri, List<int> bytes) {
-    scan(bytes);
-    byteCount += bytes.length - 1;
-  });
-  sw.stop();
-  print("Scanning files took: ${sw.elapsed}");
-  print("Bytes/ms: ${byteCount/sw.elapsedMilliseconds}");
-}
-
-main(List<String> arguments) {
-  Map<Uri, List<int>> files = <Uri, List<int>>{};
-  Stopwatch sw = new Stopwatch()..start();
-  for (String name in arguments) {
-    Uri uri = Uri.base.resolve(name);
-    List<int> bytes = readBytesFromFileSync(uri);
-    files[uri] = bytes;
-  }
-  sw.stop();
-  print("Reading files took: ${sw.elapsed}");
-  scanAll(files);
-}