Refactor command code and add support for --help and --version.

utils/pub/package.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.
 
 /**
  * A named, versioned, unit of code and resource reuse.
  */
 class Package implements Hashable {
   /**
    * Loads the package whose root directory is [packageDir].
@@ skipping 28 matching lines @@
    * Constructs a package. This should not be called directly. Instead, acquire
    * packages from [load()].
    */
   Package._(String dir, this.dependencies)
   : dir = dir,
     name = basename(dir);
 
   /**
    * Reads and returns all of the packages this package immediately depends on.
    */
-  Future<Collection<Package>> loadDependencies() {
+  Future<Collection<Package>> loadDependencies(PackageCache cache) {
     return Futures.wait(dependencies.map((name) => cache.find(name)));
   }
 
   /**
    * Walks the entire dependency graph starting at this package and returns a
    * [Set] of all of the packages dependend on by this one, directly or
    * indirectly. This package is included in the result set.
    */
-  Future<Set<Package>> traverseDependencies() {
+  Future<Set<Package>> traverseDependencies(PackageCache cache) {
     final completer = new Completer<Set<Package>>();
     final packages = new Set<Package>();
 
     var pendingAsyncCalls = 0;
 
     walkPackage(Package package) {
       // Skip packages we've already traversed.
       if (packages.contains(package)) return;
 
       // Add the package.
@@ skipping 64 matching lines @@
         completer.completeException(
             'The pubspec dependencies must be a list of package names.');
       }
 
       completer.complete(dependencies);
     });
 
     return completer.future;
   }
 }