Make mappedBy lazy.

utils/pub/hosted_source.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 hosted_source;
 
 import 'dart:async';
 import 'dart:io' as io;
 import 'dart:json' as json;
 import 'dart:uri';
@@ skipping 22 matching lines @@
   /**
    * Downloads a list of all versions of a package that are available from the
    * site.
    */
   Future<List<Version>> getVersions(String name, description) {
     var parsed = _parseDescription(description);
     var fullUrl = "${parsed.last}/packages/${parsed.first}.json";
 
     return httpGetString(fullUrl).transform((body) {
       var doc = json.parse(body);
-      return doc['versions'].mappedBy((version) => new Version.parse(version));
+      return doc['versions']
+          .mappedBy((version) => new Version.parse(version))
+          .toList();
     }).transformException((ex) {
       _throwFriendlyError(ex, parsed.first, parsed.last);
     });
   }
 
   /**
    * Downloads and parses the pubspec for a specific version of a package that
    * is available from the site.
    */
   Future<Pubspec> describe(PackageId id) {
@@ skipping 110 matching lines @@
 
     var name = description["name"];
     if (name is! String) {
       throw new FormatException("The 'name' key must have a string value.");
     }
 
     var url = description.containsKey("url") ? description["url"] : defaultUrl;
     return new Pair<String, String>(name, url);
   }
 }