Add support for pub install.

utils/pub/system_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 system-wide cache of installed packages.
+ *
+ * This cache contains all packages that are downloaded from the internet.
+ * Packages that are available locally (e.g. from the SDK) don't use this cache.
+ */
+class SystemCache {
+  /**
+   * The root directory where this package cache is located.
+   */
+  final String rootDir;
+
+  /**
+   * Packages which are currently being asynchronously installed to the cache.
+   */
+  final Map<PackageId, Future<Package>> _pendingInstalls;
+
+  /**
+   * Creates a new package cache which is backed by the given directory on the
+   * user's file system.
+   */
+  SystemCache(this.rootDir)
+  : _pendingInstalls = new Map<PackageId, Future<Package>>();
+
+  /**
+   * Loads all of the package ids in the cache and returns them.
+   */
+  Future<List<PackageId>> listAll() {
+    return listDir(rootDir).chain((paths) {
+      final sources = paths.map((path) {
+        final source = Source.fromName(basename(path));
+        return listDir(path).transform((subpaths) {
+          return subpaths.map((subpath) =>
+            new PackageId(basename(subpath), source));
+        });
+      });
+      return Futures.wait(sources).transform(flatten);
+    });
+  }
+
+  /**
+   * Ensures that the package identified by [id] is installed to the cache,
+   * loads it, and returns it.
+   *
+   * It is an error to try installing a package from a source with `shouldCache
+   * == false` to the system cache.
+   */
+  Future<Package> install(PackageId id) {
+    if (!id.source.shouldCache) {
+      throw new IllegalArgumentException("Package $id is not cacheable.");
+    }
+
+    var pending = _pendingInstalls[id];
+    if (pending != null) return pending;
+
+    var path = join(rootDir, id.source.name, id.source.packageName(id));
+    var future = exists(path).chain((exists) {
+      // TODO(nweiz): better error handling
+      if (exists) throw 'Package $id is already installed.';
+      return ensureDir(dirname(path));
+    }).chain((_) {
+      return id.source.install(id, path);
+    }).chain((found) {
+      if (!found) {
+        throw 'Package ${id.fullName} not found in source "${id.source.name}".';
+      }
+      return Package.load(path);
+    });
+
+    always(future, () => _pendingInstalls.remove(id));
+    _pendingInstalls[id] = future;
+    return future;
+  }
+}