unnecessary_string_interpolations  
            
            Unnecessary string interpolation.
This rule is available as of Dart 2.8.1.
Rule sets: recommended, flutter
This rule has a quick fix available.
Details
DON’T use string interpolation if there’s only a string expression in it.
BAD:
String message;
String o = '$message';
GOOD:
String message;
String o = message;
Usage
To enable the unnecessary_string_interpolations rule,
add unnecessary_string_interpolations under linter > rules in your
analysis_options.yaml
file:
linter:
  rules:
    - unnecessary_string_interpolations