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

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

Index: sdk/lib/_internal/pub/test/cache/add/unexpected_arguments_test.dart
diff --git a/sdk/lib/_internal/pub/test/cache/add/unexpected_arguments_test.dart b/sdk/lib/_internal/pub/test/cache/add/unexpected_arguments_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..2ba1e67a55c93181c93c1084ce3c0f0b240e7e7e
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+++ b/sdk/lib/_internal/pub/test/cache/add/unexpected_arguments_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 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);
+  });
+}