Wrong cli flags when set mainFields in JS API

[yoyo930021]

await service.build({
  entryPoints: [getServerURL('src/main.ts')],
  outfile: getServerURL('dist/vls.js'),
  minify: true,
  bundle: true,
  sourcemap: true,
  platform: 'node',
  mainFields: ['module,main'],
  target: 'es2018',
  external: ['vscode', 'eslint-plugin-vue', 'stylus', 'eslint'],
  format: 'cjs',
  tsconfig: getServerURL('tsconfig.json')
})

Error: Build failed with 2 errors:
error: Invalid file extension: "main"
error: Invalid file extension: "module"
    at failureErrorWithLog (/Users/yoyo930021/git/vetur/node_modules/esbuild/lib/main.js:628:15)
    at /Users/yoyo930021/git/vetur/node_modules/esbuild/lib/main.js:492:31
    at handleIncomingPacket (/Users/yoyo930021/git/vetur/node_modules/esbuild/lib/main.js:474:9)
    at Socket.readFromStdout (/Users/yoyo930021/git/vetur/node_modules/esbuild/lib/main.js:417:7)
    at Socket.emit (events.js:315:20)
    at addChunk (_stream_readable.js:295:12)
    at readableAddChunk (_stream_readable.js:271:9)
    at Socket.Readable.push (_stream_readable.js:212:10)
    at Pipe.onStreamRead (internal/stream_base_commons.js:186:23)