If the replacement table or function results in a value that isn't a
string or number (or nil), string.gsub raises an error. Have ustring
raise the same error.
Bug: T195326
Change-Id: Ic36f9f5d7adc0c14e7a4a94d3747335107acd8b6
Manually import LuaSandbox's git repository as a composer dependency to
provide the PHP stubs for phan.
Change-Id: I6226b9211f31d829da5a2775c6f5cf3599dd8ebc