Support loading transformer plugins from pub.

sdk/lib/_internal/pub/lib/src/pub_package_provider.dart

-// Copyright (c) 2013, 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.pub_package_provider;
-
-import 'dart:async';
-
-import 'package:barback/barback.dart';
-import 'package:path/path.dart' as path;
-
-import 'entrypoint.dart';
-
-/// An implementation of barback's [PackageProvider] interface so that barback
-/// can assets within pub packages.
-class PubPackageProvider implements PackageProvider {
-  /// Maps the names of all of the packages in [_entrypoint]'s transitive
-  /// dependency graph to the local path of the directory for that package.
-  final Map<String, String> _packageDirs;
-
-  /// Creates a new provider for [entrypoint].
-  static Future<PubPackageProvider> create(Entrypoint entrypoint) {
-    var packageDirs = <String, String>{};
-
-    packageDirs[entrypoint.root.name] = entrypoint.root.dir;
-
-    // Cache package directories up front so we can have synchronous access
-    // to them.
-    var futures = [];
-    entrypoint.loadLockFile().packages.forEach((name, package) {
-      var source = entrypoint.cache.sources[package.source];
-      futures.add(source.getDirectory(package).then((packageDir) {
-        packageDirs[name] = packageDir;
-      }));
-    });
-
-    return Future.wait(futures).then((_) {
-      return new PubPackageProvider._(packageDirs);
-    });
-  }
-
-  PubPackageProvider._(this._packageDirs);
-
-  Iterable<String> get packages => _packageDirs.keys;
-
-  /// Gets the root directory of [package].
-  String getPackageDir(String package) => _packageDirs[package];
-
-  Future<Asset> getAsset(AssetId id) {
-    var file = path.join(_packageDirs[id.package], id.path);
-    return new Future.value(new Asset.fromPath(id, file));
-  }
-}