Assert that the string is plain text that does not have HTML, i.e. not
affected by HTML escaping. Otherwise, this raises an error if assertions are
enabled. It does NOT sanitize nor make any change to the input string. It
should only be used when the assertion failure is benign, such as printing
spurious tags. DO NOT count on this to remove unsafe HTML. It is only meant
for legacy refactoring. Please do NOT use in new code.