hasDifName = (argDatum) => {
      // The field name (aka id) is always equal to the arg id.
      return (
        argDatum.argId == this.name ||
        caselessName != argDatum.model.name.toLowerCase()
      );
    }
Returns true if the given argDatum is associated with this field, or has
a different caseless name than the argDatum associated with this field.
@param {{model: Blockly.VariableModel, argId:string}} argDatum The
    argDatum we want to make sure does not conflict with the argDatum
    associated with this field.
@returns {boolean} True if the given datum does not conflict with the
    datum associated with this field.
@this {Blockly.FieldTextInput}