No Adjacent Strings in lists

[a14n]

It can be easy to forget a comma in String list without error because it's still valid code. In String list we should use + operator to prevent this.

BAD

var list = <String>[
  'a'
  'b',
  'c',
];

GOOD

var list = <String>[
  'a' +
  'b',
  'c',
];