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

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

Index: sdk/lib/_internal/pub/test/cache/add/package_not_found_test.dart
diff --git a/sdk/lib/_internal/pub/test/cache/add/package_not_found_test.dart b/sdk/lib/_internal/pub/test/cache/add/package_not_found_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..9e60b5f7790e88b3d365cc37e645e0d02c10310d
--- /dev/null
+++ b/sdk/lib/_internal/pub/test/cache/add/package_not_found_test.dart
@@ -0,0 +1,18 @@
+// 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 '../../test_pub.dart';
+
+main() {
+  initConfig();
+  integration('fails if the package cound not be found on the source', () {
+    servePackages([]);
+
+    schedulePub(args: ["cache", "add", "foo"],
+        error: new RegExp(r"Could not find package foo at http://.*"),
+        exitCode: 1);
+  });
+}