Start pulling pub from its own repo.

sdk/lib/_internal/pub/lib/src/validator/executable.dart

-// Copyright (c) 2012, 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 pub.validator.executable;
-
-import 'dart:async';
-
-import 'package:path/path.dart' as p;
-
-import '../entrypoint.dart';
-import '../validator.dart';
-
-/// Validates that a package's pubspec doesn't contain executables that
-/// reference non-existent scripts.
-class ExecutableValidator extends Validator {
-  ExecutableValidator(Entrypoint entrypoint)
-    : super(entrypoint);
-
-  Future validate() async {
-    // TODO(rnystrom): This can print false positives since a script may be
-    // produced by a transformer. Do something better.
-    var binFiles = entrypoint.root.listFiles(beneath: "bin", recursive: false)
-        .map((path) => entrypoint.root.relative(path))
-        .toList();
-
-    entrypoint.root.pubspec.executables.forEach((executable, script) {
-      var scriptPath = p.join("bin", "$script.dart");
-      if (binFiles.contains(scriptPath)) return;
-
-      warnings.add('Your pubspec.yaml lists an executable "$executable" that '
-          'points to a script "$scriptPath" that does not exist.');
-    });
-  }
-}