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

sdk/lib/_internal/pub/test/cache/add/already_cached_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 '../../descriptor.dart' as d;
+import '../../test_pub.dart';
+
+main() {
+  initConfig();
+  integration('does nothing if the package is already cached', () {
+    servePackages([
+      packageMap("foo", "1.2.3")
+    ]);
+
+    // Run once to put it in the cache.
+    schedulePub(args: ["cache", "add", "foo"],
+        output: 'Downloading foo 1.2.3...');
+
+    // Should be in the cache now.
+    schedulePub(args: ["cache", "add", "foo"],
+        output: 'Already cached foo 1.2.3.');
+
+    d.cacheDir({"foo": "1.2.3"}).validate();
+  });
+}