constructor(code, filename) {
    this._code = String(code);
    this.filename = filename || 'vm.js';
    this._prefix = '';
    this._suffix = '';
    this._compiledVM = null;
    this._compiledNodeVM = null;
  }
Create VMScript instance.

@param {String} code Code to run.
@param {String} [filename] Filename that shows up in any stack traces produced from this script.
@return {VMScript}