Add support for pub install.

utils/pub/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.
-
-/**
- * The local cache of previously installed packages.
- */
-class PackageCache {
-  /**
-   * The root directory where this package cache is located.
-   */
-  final String rootDir;
-
-  // TODO(rnystrom): When packages are versioned, String here and elsewhere will
-  // become a name/version/(source?) tuple.
-  final Map<String, Package> _loadedPackages;
-
-  /**
-   * Packages which are currently being asynchronously loaded.
-   */
-  final Map<String, Future<Package>> _pendingPackages;
-
-  /**
-   * Creates a new package cache which is backed by the given directory on the
-   * user's file system.
-   */
-  PackageCache(this.rootDir)
-  : _loadedPackages = <Package>{},
-    _pendingPackages = <Future<Package>>{};
-
-  /**
-   * Loads all of the packages in the cache and returns them.
-   */
-  Future<List<Package>> listAll() {
-    return listDir(rootDir).chain((paths) {
-      final packages = paths.map((path) => find(basename(path)));
-      return Futures.wait(packages);
-    });
-  }
-
-  /**
-   * Loads the package named [name] from this cache, if present.
-   */
-  // TODO(rnystrom): What happens if the package isn't cached?
-  Future<Package> find(String name) {
-    // Use the previously loaded one.
-    final package = _loadedPackages[name];
-    if (package != null) return new Future.immediate(package);
-
-    // If we are already in-progress loading it, re-use that one.
-    final pending = _pendingPackages[name];
-    if (pending != null) return pending;
-
-    // Actually load it from the cache.
-    final future = Package.load(join(rootDir, name)).transform((package) {
-      _pendingPackages.remove(name);
-      _loadedPackages[name] = package;
-      return package;
-    });
-
-    _pendingPackages[name] = future;
-    return future;
-  }
-}