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

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

Index: sdk/lib/_internal/pub/test/cache/add/missing_package_arg_test.dart
diff --git a/sdk/lib/_internal/pub/test/cache/add/missing_package_arg_test.dart b/sdk/lib/_internal/pub/test/cache/add/missing_package_arg_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..e95ede750b4243b4841bbe66d6a6bb8cf302a0fe
--- /dev/null
+++ b/sdk/lib/_internal/pub/test/cache/add/missing_package_arg_test.dart
@@ -0,0 +1,24 @@
+// 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 no package was given', () {
+    schedulePub(args: ["cache", "add"],
+        error: """
+            No package to add given.
+            
+            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);
+  });
+}