createNode(workspace) {
    if (!this.hasValidPosition()) {
      return null;
    }
    if (this.type == 'BLOCK') {
      return this.createBlockNode_(workspace);
    } else if (this.type == 'FIELD') {
      return this.createFieldNode_(workspace);
    }
  }
Create an ASTNode pointing to the Position.
@param {!Blockly.Workspace} workspace The workspace the user is on.
@returns {Blockly.ASTNode} An AST Node that points to the Position or null
if the position is not viable.
@public