Refactor command code and add support for --help and --version.

utils/pub/command_update.dart

Index: utils/pub/command_update.dart
diff --git a/utils/pub/command_update.dart b/utils/pub/command_update.dart
index 27e04b5ea30ff5e0862d0cbab482924433bca842..a348f3f9a082dc69c2d0a2912e74b55275f3a6ba 100644
--- a/utils/pub/command_update.dart
+++ b/utils/pub/command_update.dart
@@ -3,25 +3,28 @@
 // BSD-style license that can be found in the LICENSE file.
 
 /** Handles the `update` pub command. */
-void commandUpdate(PubOptions options, List<String> args) {
-  // TODO(rnystrom): Should validate args.
+class UpdateCommand extends PubCommand {
+  String get description() =>
+    "update the current package's dependencies to the latest versions";
 
-  var entrypoint;
-  var dependencies;
-  var packagesDir;
+  void onRun() {
+    var entrypoint;
+    var packagesDir;
+    var dependencies;
 
-  getWorkingPackage().chain((package) {
-    entrypoint = package;
-    return package.traverseDependencies();
-  }).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));
-    }
-  });
+    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));
+      }
+    });
+  }
 }