Add support for pub install.

utils/pub/command_update.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 `update` pub command. */
-class UpdateCommand extends PubCommand {
+// TODO(nweiz): Make update do something different than install when we have
+// sources for which that makes sense.
+class UpdateCommand extends InstallCommand {
   String get description() =>
     "update the current package's dependencies to the latest versions";
-
-  void onRun() {
-    var entrypoint;
-    var packagesDir;
-    var dependencies;
-
-    getWorkingPackage().chain((package) {
-      entrypoint = package;
-      return package.traverseDependencies(cache);
-    }).chain((packages) {
-      dependencies = packages;
-      // TODO(rnystrom): Make this path configurable.
-      packagesDir = join(entrypoint.dir, 'packages');
-      return cleanDir(packagesDir);
-    }).then((dir) {
-      // Symlink each dependency.
-      for (final package in dependencies) {
-        createSymlink(package.dir, join(packagesDir, package.name));
-      }
-    });
-  }
 }