用gulp 编译打包typescript后提示命名空间未定义？

Question

//index.ts 入口文件

let app:Test.App
window.addEventListener('DOMContentLoaded',()=>{
   app = new Test.App()
})

// app.ts

namespace Test{
  export class App{
    public a:number= 1
    constructor(){
      this.a = 100
    }
    public run () {
      console.log('1212')
    }
  }
}

//gulpfiles

gulp.task("copy-html", function () {
    let stream = gulp.src(['src/*.html'])
    return stream.pipe(gulp.dest("dist"))
});

let tasks = ["copy-html"]
gulp.task("default",gulp.series(tasks,function(){
  return browserify({
    basedir: '.',
    debug: true,
    entries: ['src/index.ts','src/app.ts'],
    cache: {},
    packageCache: {}
})
.plugin(tsify)
.bundle()
.pipe(source('bundle.js'))
.pipe(gulp.dest("dist"));

}))

运行时提示 Test未定义