function makeField(id, tag = undefined) {
  const field = new Field(id);
  field.registerPlugin(new TagOnEnterHandler(tag || TagName.P));
  return field;
}
Creates a field with only the enter handler plugged in, for testing.
@param {string} id A DOM id.
@param {!TagName=} tag The block tag to use. Defaults to P.
@return {!Field} A field.