remove use of Options class

It seems there no longer is an options Class in the dart language. As argParser expects a List<String> as an argument, it is appropriate to replace Options with the args passed to main.
remove imports for 'dart:json' and 'dart:utf'

These modules seem to have seased to exist. As these modules are not used in the concerned files, the imports are simply dropped.
remove unused imports to avoid warnings
reformulate to remove warning

The DartEditor did not understand 'other = other as _User' and threw a warning. As a means, we intorduce a new variable otherUser.