code: function compile(cmdLine, options) {
    return new rsvp.Promise(function(resolve, reject) {
      exec(cmdLine, options, function(err, stdout, stderr) {
        if (err) {
          // Provide a robust error message in case of failure.
          // compass sends errors to sdtout, so it's important to include that
          err.message = '\n[broccoli-compass] ' + err +
                        '    directory: ' + options.cwd + '\n' +
                        '    cmdline: ' + cmdLine + '\n' +
                        '    stderr: ' + stderr + '\n' +
                        (stdout && '    stdout:  ' + stdout + '\n' || '\n');
  
          return reject(err);
        }
  
        resolve();
      });
    });
  }
docs: TODO: collect sass/scss on construct to build the list css generated files for copy. 
  Executes the cmdLine statement in a Promise.
  @param cmdLine   The compass compile command line statement.
  @param options   The options for exec.
  @returns {exports.Promise}