getMostRestrictiveKind() {
    const kindToVal = {
      'var': 1,
      'let': 2,
      'const': 3,
    };
    const valToKind = {
      1: 'var',
      2: 'let',
      3: 'const',
    };

    const minVal = min(this.variables.map(v => kindToVal[v.getKind()]));
    return valToKind[minVal];
  }
Returns the most restrictive possible common `kind` value
for variables defined in this group.

- When all vars are const, return "const".
- When some vars are "let" and some "const", returns "let".
- When some vars are "var", return "var".

@return {String} Either "var", "let" or "const".