First stab at a dev server in pub using barback.

sdk/lib/_internal/pub/lib/src/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 pub.source;
 
 import 'dart:async';
 
 import 'package:path/path.dart' as path;
 
@@ skipping 166 matching lines @@
   bool _isCachedPackageCorrupted(String packageDir) {
     if (!fileExists(path.join(packageDir, "pubspec.yaml"))) return true;
 
     var libDir = path.join(packageDir, "lib");
     if (dirExists(libDir)) return listDir(libDir).length == 0;
 
     // If we got here, it's OK.
     return false;
   }
 
+  /// Returns the directory where this package has been installed. If this is
+  /// a cached source, it will be in the system cache. Otherwise, it will
+  /// depend on the source.
+  Future<String> getDirectory(PackageId id) {
+    if (shouldCache) return systemCacheDirectory(id);
+    throw new UnimplementedError("Source $name must implement this.");
+  }
+
   /// Returns the directory in the system cache that the package identified by
   /// [id] should be installed to. This should return a path to a subdirectory
   /// of [systemCacheRoot].
   ///
   /// This doesn't need to be implemented if [shouldCache] is false.
   Future<String> systemCacheDirectory(PackageId id) {
     return new Future.error(
         "systemCacheDirectory() must be implemented if shouldCache is true.");
   }
 
@@ skipping 50 matching lines @@
   /// Returns the [Package]s that have been installed in the system cache.
   List<Package> getCachedPackages() {
     if (shouldCache) {
       throw new UnimplementedError("Source $name must implement this.");
     }
   }
 
   /// Returns the source's name.
   String toString() => name;
 }