Move pubspec caching into each source.

lib/src/command/cache_add.dart

 // Copyright (c) 2014, 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 pub.command.cache_add;
 
 import 'dart:async';
 
 import 'package:pub_semver/pub_semver.dart';
 
@@ skipping 41 matching lines @@
         constraint = new VersionConstraint.parse(argResults["version"]);
       } on FormatException catch (error) {
         usageException(error.message);
       }
     }
 
     // TODO(rnystrom): Support installing from git too.
     var source = cache.sources["hosted"];
 
     // TODO(rnystrom): Allow specifying the server.
-    var pubspecs = await source.getVersions(package, package);
-    var versions = pubspecs.map((pubspec) => pubspec.version)
+    var ids = await source.getVersions(
+        new PackageRef(package, 'hosted', package));
+    var versions = ids.map((id) => id.version)
         .where(constraint.allows).toList();
 
     if (versions.isEmpty) {
       // TODO(rnystrom): Show most recent unmatching version?
       fail("Package $package has no versions that match $constraint.");
     }
 
     downloadVersion(version) async {
       var id = new PackageId(package, source.name, version, package);
       if (cache.contains(id)) {
@@ skipping 11 matching lines @@
       // Install them in ascending order.
       versions.sort();
       await Future.forEach(versions, downloadVersion);
     } else {
       // Pick the best matching version.
       versions.sort(Version.prioritize);
       await downloadVersion(versions.last);
     }
   }
 }