Fix: Remove HMR attempts in Prod~!

[lukeed]

In production apps, there were still attempts made to /__webpack_hmr, which meant that the dev-entry was being included.

The short-fix was to modify the build script: NODE_ENV=production sapper build ... but that's still ugly & not the intended.

I added some debuggers and saw that the dev block of create_app was still running. Then took a look at the compiled cli.js output:

The dev check is made before manually setting the NODE_ENV. Also, because of Rollup's "import hoisting" (is that the term?) moving this line to the top of the file, ahead of the dev import, didn't affect the compiled output.

Simply converting config.dev to config.isDev() solves this, deferring the check until needed.

Now, sapper build && sapper start works as expected, while preserving HMR for dev-mode. 🎉

[Rich-Harris]

I wish I'd thunk of this

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[tumbleweed]