Add a Pub source for pub.dartlang.org.

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('io.dart');
 #import('package.dart');
 #import('source_registry.dart');
+#import('utils.dart');
 
 /**
  * The system-wide cache of installed packages.
  *
  * This cache contains all packages that are downloaded from the internet.
  * Packages that are available locally (e.g. from the SDK) don't use this cache.
  */
 class SystemCache {
   /**
    * The root directory where this package cache is located.
@@ skipping 53 matching lines @@
 
     var sourceDir = join(rootDir, id.source.name);
     var path = id.source.systemCacheDirectory(id, sourceDir);
     var future = exists(path).chain((exists) {
       // TODO(nweiz): better error handling
       if (exists) throw 'Package $id is already installed.';
       return ensureDir(dirname(path));
     }).chain((_) {
       return id.source.install(id, path);
     }).chain((found) {
-      if (!found) {
-        throw 'Package ${id.fullName} not found in source "${id.source.name}".';
-      }
+      if (!found) throw 'Package $id not found.';
       return Package.load(path, sources);
     });
 
     always(future, () => _pendingInstalls.remove(id));
     _pendingInstalls[id] = future;
     return future;
   }
 }