GetPreferredUnits asserts constraints on its region argument instead of expressing them in the parameter type #151

GetPreferredUnits declares its region_ parameter as just "a String", then asserts that it matches the unicode_region_id Unicode locale nonterminal and equals its own ASCII-uppercase. Constraints like these belong in the parameter's type, not in a runtime assertion (raised in #110 (comment)). The value comes from GetLocaleRegion, which 402 itself types as returning just a String.