Add validation infrastructure for "pub lish".

utils/pub/entrypoint.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 entrypoint;
 
 import 'io.dart';
 import 'lock_file.dart';
 import 'package.dart';
 import 'root_source.dart';
@@ skipping 33 matching lines @@
 
   /**
    * Packages which are either currently being asynchronously installed to the
    * directory, or have already been installed.
    */
   final Map<PackageId, Future<PackageId>> _installs;
 
   Entrypoint(this.root, this.cache)
   : _installs = new Map<PackageId, Future<PackageId>>();
 
+  /// Loads the entrypoint from a package at [rootDir].
+  static Future<Entrypoint> load(String rootDir, SystemCache cache) {
+    return Package.load(null, rootDir, cache.sources).transform((package) =>
+        new Entrypoint(package, cache));
+  }
+
   /**
    * The path to this "packages" directory.
    */
   // TODO(rnystrom): Make this path configurable.
   String get path => join(root.dir, 'packages');
 
   /**
    * Ensures that the package identified by [id] is installed to the directory.
    * Returns the resolved [PackageId].
    *
@@ skipping 209 matching lines @@
    * Creates a symlink to the `packages` directory in [dir] if none exists.
    */
   Future _linkSecondaryPackageDir(String dir) {
     var to = join(dir, 'packages');
     return exists(to).chain((exists) {
       if (exists) return new Future.immediate(null);
       return createSymlink(path, to);
     });
   }
 }