Use the cached pubspec if possible for describing hosted packages.

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 31 matching lines @@
           .toList();
     }).catchError((ex) {
       var parsed = _parseDescription(description);
       _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) {
+    // Request it from the server.
     var url = _makeVersionUrl(id, (server, package, version) =>
         "$server/packages/$package/versions/$version.yaml");
 
     log.io("Describe package at $url.");
     return httpClient.read(url).then((yaml) {
+      // TODO(rnystrom): After this is pulled down, we could place it in
+      // a secondary cache of just pubspecs. This would let us have a
+      // persistent cache for pubspecs for packages that haven't actually
+      // been installed.
       return new Pubspec.parse(null, yaml, systemCache.sources);
     }).catchError((ex) {
       var parsed = _parseDescription(id.description);
       _throwFriendlyError(ex, id, parsed.last);
     });
   }
 
   /// Downloads a package from the site and unpacks it.
   Future<bool> install(PackageId id, String destPath) {
     return new Future.sync(() {
@@ skipping 48 matching lines @@
   dynamic parseDescription(String containingPath, description,
                            {bool fromLockFile: false}) {
     _parseDescription(description);
     return description;
   }
 
   List<Package> getCachedPackages() {
     var cacheDir = path.join(systemCacheRoot,
                              _getSourceDirectory(_defaultUrl));
     if (!dirExists(cacheDir)) return [];
-  
+
     return listDir(path.join(cacheDir)).map((entry) =>
         new Package.load(null, entry, systemCache.sources)).toList();
   }
-  
+
   /// When an error occurs trying to read something about [package] from [url],
   /// this tries to translate into a more user friendly error message. Always
   /// throws an error, either the original one or a better one.
   void _throwFriendlyError(error, package, url) {
     if (error is PubHttpException &&
         error.response.statusCode == 404) {
       fail('Could not find package "$package" at $url.');
     }
 
     if (error is TimeoutException) {
@@ skipping 60 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);
 }