First stab at a dev server in pub using barback.

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 'dart:io';
+
+import 'package:barback/barback.dart';
+import 'package:path/path.dart' as path;
+
+import 'entrypoint.dart';
+import 'io.dart';
+
+/// An implementation of barback's [PackageProvider] interface so that barback
+/// can assets within pub packages.
+class PubPackageProvider implements PackageProvider {
+  /// The [Entrypoint] package being served.
+  final Entrypoint _entrypoint;
+
+  /// 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.
+    // TODO(rnystrom): Handle missing or out of date lockfile.
+    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._(entrypoint, packageDirs);
+    });
+  }
+
+  PubPackageProvider._(this._entrypoint, this._packageDirs);
+
+  Iterable<String> get packages => _packageDirs.keys;
+
+  /// Lists all of the visible files in [package].
+  ///
+  /// This is the recursive contents of the "asset" and "lib" directories (if
+  /// present). If [package] is the entrypoint package, it also includes the
+  /// contents of "web".
+  List<AssetId> listAssets(String package) {
+    var files = <AssetId>[];
+
+    addFiles(String dirPath) {
+      var packageDir = _packageDirs[package];
+      var dir = path.join(packageDir, dirPath);
+      if (!dirExists(dir)) return;
+      var entries = listDir(dir, recursive: true);
+      files.addAll(entries
+          .where((entry) => !path.split(entry).contains("packages"))
+          .where(fileExists)
+          .map((entry) => new AssetId(package,
+              path.relative(entry, from: packageDir))));
+    }
+
+    // Expose the "asset" and "lib" directories.
+    addFiles("asset");
+    addFiles("lib");
+
+    // The entrypoint's "web" directory is also visible.
+    if (package == _entrypoint.root.name) {
+      addFiles("web");
+    }
+
+    return files;
+  }
+
+  // TODO(rnystrom): Actually support transformers.
+  Iterable<Iterable<Transformer>> getTransformers(String package) => [];
+
+  Future<Asset> getAsset(AssetId id) {
+    var file = path.join(_packageDirs[id.package], id.path);
+    return new Future.value(new Asset.fromPath(id, file));
+  }
+}