Use real "package:" imports in pub's plugin-loading code.

sdk/lib/_internal/pub/lib/src/barback/build_environment.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.barback.build_environment;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:barback/barback.dart';
@@ skipping 210 matching lines @@
         .any((transformers) => transformers
         .any((id) => id.package == '\$dart2js'));
 
     if (!containsDart2JS && useDart2JS) {
       _builtInTransformers.addAll([
         new Dart2JSTransformer(this, mode),
         new DartForwardingTransformer(mode)
       ]);
     }
 
+    // "$pub" is a psuedo-package that allows pub's transformer-loading
+    // infrastructure to share code with pub proper. We provide it only during
+    // the initial transformer loading process.
+    var dartPath = assetPath('dart');
+    var pubSources = listDir(dartPath).map((library) {
+      return new AssetId('\$pub',
+          path.join('lib', path.relative(library, from: dartPath)));
+    });
+
     // Bind a server that we can use to load the transformers.
     var transformerServer;
     return BarbackServer.bind(this, _hostname, 0, null).then((server) {
       transformerServer = server;
 
-      return log.progress("Loading source assets", _provideSources);
+      return log.progress("Loading source assets", () {
+        barback.updateSources(pubSources);
+        return _provideSources();
+      });
     }).then((_) {
       log.fine("Provided sources.");
       var completer = new Completer();
 
       // If any errors get emitted either by barback or by the transformer
       // server, including non-programmatic barback errors, they should take
       // down the whole program.
       var subscriptions = [
         barback.errors.listen((error) {
           if (error is TransformerException) {
@@ skipping 28 matching lines @@
         if (!completer.isCompleted) {
           completer.completeError(error, stackTrace);
         }
       });
 
       return completer.future.whenComplete(() {
         for (var subscription in subscriptions) {
           subscription.cancel();
         }
       });
-    });
+    }).then((_) => barback.removeSources(pubSources));
   }
 
   /// Provides all of the source assets in the environment to barback.
   ///
   /// If [watcherType] is not [WatcherType.NONE], enables watching on them.
   Future _provideSources() {
     return Future.wait(graph.packages.values.map((package) {
       return Future.wait(_getPublicDirectories(package.name)
           .map((dir) => _provideDirectorySources(package, dir)));
     }));
@@ skipping 214 matching lines @@
   String toString() => "polling";
 }
 
 class _NoneWatcherType implements WatcherType {
   const _NoneWatcherType();
 
   DirectoryWatcher create(String directory) => null;
 
   String toString() => "none";
 }