First pass at "pub update" implementation.

utils/pub/pub.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 main entrypoint for the pub command line application.
  */
 #library('pub');
 
 #import('io.dart');
 #import('utils.dart');
 
 #source('cache.dart');
 #source('command_list.dart');
 #source('command_update.dart');
 #source('package.dart');
 
 Map<String, PubCommand> commands;
+PackageCache cache;
 
 main() {
   final args = new Options().arguments;
 
   // TODO(rnystrom): In addition to explicit "help" and "version" commands,
   // should also add special-case support for --help and --version arguments to
   // be consistent with other Unix apps.
   commands = {
     'list': new PubCommand('print the contents of repositories', commandList),
+    'update': new PubCommand("update a package's dependencies", commandUpdate),
     'version': new PubCommand('print Pub version', commandVersion)
   };
 
   if (args.length == 0) {
     showUsage();
     return;
   }
 
   // TODO(rnystrom): Hack. This is temporary code to allow the pub tests to
   // pass in relevant paths. Eventually these should be either environment
   // variables or at least a cleaner arg parser.
   var cacheDir;
   for (var i = 0; i < args.length; i++) {
     if (args[i].startsWith('--cachedir=')) {
       cacheDir = args[i].substring('--cachedir='.length);
       args.removeRange(i, 1);
       break;
     }
   }
 
+  // TODO(rnystrom): Do we want this to be global?
+  cache = new PackageCache(cacheDir);
+
   var options = new PubOptions(cacheDir);
 
   // Select the command.
   final command = commands[args[0]];
   if (command == null) {
     print('Unknown command "${args[0]}".');
     print('Run "pub help" to see available commands.');
     exit(64); // See http://www.freebsd.org/cgi/man.cgi?query=sysexits.
     return;
   }
@@ skipping 31 matching lines @@
   print('');
   print('Use "pub help [command]" for more information about a command.');
 }
 
 /** Displays pub version information. */
 void commandVersion(PubOptions options, List<String> args) {
   // TODO(rnystrom): Store some place central.
   print('Pub 0.0.0');
 }
 
+/**
+ * Gets the package that contains the current working directory. In other words,
+ * finds the package that the user is currently "in".
+ */
+Future<Package> getWorkingPackage() {
+  // TODO(rnystrom): Will eventually need better logic to walk up
+  // subdirectories until we hit one that looks package-like. For now, just
+  // assume the cwd is it.
+  return Package.load(workingDir);
+}
+
 typedef void CommandFunction(PubOptions options, List<String> args);
 
 class PubCommand {
   final String description;
   final CommandFunction function;
 
   PubCommand(this.description, this.function);
 }
 
 class PubOptions {
   final String cacheDir;
 
   PubOptions(this.cacheDir);
 }