On my system "gulp watch" was failing to generate the client's entry
point script `build/boot.js` correctly. This may have happened after
recent upgrades to gulp or Node.

The issue was that `generateManifest` was run after "gulp
watch" started, before build/scripts/boot.bundle.js was generated,
producing an empty output file in build/boot.js. It was not re-run later
after build/scripts/boot.bundle.js was generated.

After some experimentation, removing use of `gulp-batch` in the
`watch-manifest` task resolved the issue. I haven't verified whether the
node upgrade or a gulp upgrade was the root cause. Since gulp-batch was
non-essential, I've just removed it to simplify things and reduce the
risk of similar issues happening in future.