createFieldNode_(workspace) {
    const block = workspace.getBlockById(this.blockId);
    const field = block.getField(this.fieldName);
    return Blockly.ASTNode.createFieldNode(field);
  }
Create an ASTNode pointing to a field.
@param {!Blockly.Workspace} workspace The workspace the user is on.
@returns {Blockly.ASTNode} An AST Node that points to a field.
@public