Add support for pub install.

utils/pub/command_install.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.
+
+/** Handles the `install` pub command. */
+class InstallCommand extends PubCommand {
+  String get description() => "install the current package's dependencies";
+
+  void onRun() {
+    getWorkingPackage().chain((pkg) {
+      return Futures.wait(pkg.dependencies.map(appCache.installTransitively));
+    }).then((_) {
+      print('Dependencies installed!');
+    });
+  }
+}