Use the cached pubspec if possible for describing hosted packages.

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.
 
 library package;
 
 import 'dart:async';
 
 import 'package:pathos/path.dart' as path;
 
@@ skipping 128 matching lines @@
     var sourceComp = source.name.compareTo(other.source.name);
     if (sourceComp != 0) return sourceComp;
 
     var nameComp = name.compareTo(other.name);
     if (nameComp != 0) return nameComp;
 
     return version.compareTo(other.version);
   }
 
   /// Returns the pubspec for this package.
-  Future<Pubspec> describe() => source.describe(this);
+  Future<Pubspec> describe() => source.systemCache.describe(this);
 
   /// Returns a future that completes to the resovled [PackageId] for this id.
   Future<PackageId> get resolved => source.resolveId(this);
 
   /// Returns a [PackageRef] that references this package and constrains its
   /// version to exactly match [version].
   PackageRef toRef() {
     return new PackageRef(name, source, version, description);
   }
 
@@ skipping 69 matching lines @@
 
 class PubspecNameMismatchException implements Exception {
   final String expectedName;
   final String actualName;
 
   PubspecNameMismatchException(this.expectedName, this.actualName);
 
   String toString() => 'The name you specified for your dependency, '
     '"$expectedName", doesn\'t match the name "$actualName" in its pubspec.';
 }