supports traditional way to set source and dest.

  grunt.initConfig({

    traceur: {
      options: {
          // traceur options here
          experimental: true,
          blockBinding: true,
          deferredFunctions: true,
          annotations: true,
          debug:true
      },
      custom: {
          files:[{
              expand: true,
              cwd: '<%= yeoman.app %>/modules/',
              src: ['**/*.es6.js'],
              dest: '<%= yeoman.app %>/modules/',
              ext: '.js'
          }]
      }
    },...


    // Watches files for changes and runs tasks based on the changed files
    watch: {
      traceur :  {
        files :  [ '<%= yeoman.app %>/modules/**/*.es6.js' ],
        tasks :  [ 'newer:traceur' ]
      }, ....