Resolves https://github.com/flutter/flutter/issues/138297. 

When reading from a .arb file (which contains localizations of strings in the form of a user-defined JSON string-string map), validate the type of values that we read.

See [this comment of mine on #138297](https://github.com/flutter/flutter/issues/138297#issuecomment-1827043260) to see how I arrived at this fix.