Add a "cache add" command to download packages to the cache.

sdk/lib/_internal/pub/test/cache/add/unexpected_arguments_test.dart

+// Copyright (c) 2014, 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.
+
+library pub_tests;
+
+import '../../../lib/src/exit_codes.dart' as exit_codes;
+import '../../test_pub.dart';
+
+main() {
+  initConfig();
+  integration('fails if there are extra arguments', () {
+    schedulePub(args: ["cache", "add", "foo", "bar", "baz"],
+        error: """
+            Unexpected arguments "bar" and "baz".
+            
+            Usage: pub cache add <package> [--version <constraint>] [--all]
+            -h, --help       Print usage information for this command.
+                --all        Install all matching versions.
+            -v, --version    Version constraint.
+            """,
+        exitCode: exit_codes.USAGE);
+  });
+}