code: function(callback) {
      async.waterfall([
        // 1. Resolve loose package versions.
        function(done) {
          var master = this.master;
          var packageToVersions = this.packageToVersions;
          Package.versions(master, packageToVersions, function(err, result) {
            this.packageToVersions = result;
            done(err, result);
          }.bind(this));
        }.bind(this),
  
        // 2. Dependency search.
        this.dependencySearch.bind(this),
  
        // 3. Make sure we have package and package version dirs.
        this.makedirs.bind(this),
  
        // 4. Download the world.
        this.download.bind(this),
  
        // 5. Commit the downloads to our package repository.
        this.commit.bind(this)
      ], callback);
    }
docs: Sync the packages we're watching from the master registry.
  
  @param {Function} callback invoke when done.