getCommonKind() {
    const firstKind = this.variables[0].getKind();
    if (this.variables.every(v => v.getKind() === firstKind)) {
      return firstKind;
    }
    else {
      return undefined;
    }
  }
Returns the `kind` value of variable defined in this group.

When not all variables are of the same kind, returns undefined.

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