Use the cached pubspec if possible for describing hosted packages.

utils/pub/system_cache.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 system_cache;
 
 import 'dart:io';
 import 'dart:async';
 
 import 'package:pathos/path.dart' as path;
@@ skipping 52 matching lines @@
   }
 
   /// Gets the package identified by [id]. If the package is already cached,
   /// reads it from the cache. Otherwise, requests it from the source.
   Future<Pubspec> describe(PackageId id) {
     if (id.isRoot) throw new ArgumentError("Cannot describe the root package.");
 
     // Try to get it from the system cache first.
     if (id.source.shouldCache) {
       return id.systemCacheDirectory.then((packageDir) {
-        if (!dirExists(packageDir)) return id.describe();
+        if (!fileExists(path.join(packageDir, "pubspec.yaml"))) {
+          return id.source.describe(id);
+        }
+
         return new Pubspec.load(id.name, packageDir, sources);
       });
     }
 
     // Not cached, so get it from the source.
-    return id.describe();
+    return id.source.describe(id);
   }
 
   /// Ensures that the package identified by [id] is installed to the cache,
   /// loads it, and returns it.
   ///
   /// It is an error to try installing a package from a source with `shouldCache
   /// == false` to the system cache.
   Future<Package> install(PackageId id) {
     if (!id.source.shouldCache) {
       throw new ArgumentError("Package $id is not cacheable.");
@@ skipping 17 matching lines @@
     var temp = ensureDir(tempDir);
     return io.createTempDir(path.join(temp, 'dir'));
   }
 
   /// Deletes the system cache's internal temp directory.
   void deleteTempDir() {
     log.fine('Clean up system cache temp directory $tempDir.');
     if (dirExists(tempDir)) deleteEntry(tempDir);
   }
 }