Start implementing pub object model. Rudimentary package cache class, and a simple pub command to l…

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
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index 0000000000000000000000000000000000000000..c1fe4f4816b65723c926c8684b30c3770c32da77
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+++ b/utils/pub/command_update.dart
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+// 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. */
+void commandUpdate(PubOptions options, List<String> args) {
+  throw 'not implemented yet';
+}