Fix broken pub tests.

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
index e95ede750b4243b4841bbe66d6a6bb8cf302a0fe..221411425d1f2e751138a067b9165fe66ef3c89c 100644
--- 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
@@ -18,6 +18,8 @@ main() {
             -h, --help       Print usage information for this command.
                 --all        Install all matching versions.
             -v, --version    Version constraint.
+
+            See http://dartlang.org/tools/pub/cmd/pub-cache.html for detailed documentation.
             """,
         exitCode: exit_codes.USAGE);
   });