Converts the given JSON-string
input to its corresponding object.
this was initialized with a reviver, then the parsing operation
invokes the reviver on every object or list property that has been parsed.
The arguments are the property name (String) or list index (int), and
the value is the parsed value. The return value of the reviver is used as
the value of that property instead the parsed value.
Throws FormatException if the input is not valid JSON text.
dynamic convert(String input) => _parseJson(input, _reviver);