fix: TypeError in production build when targeting ES2015

[ppvg]

Fixes #346.

This is a workaround for a bug in uglifyjs2, which can cause name collisions when a function with arguments is inlined. This can result in an unintended shadowing of a var or let, or a TypeError: Assignment to constant variable in case of a const.

I reproduced the issue by creating a new app (create-react-app --scripts-version=react-scripts-ts), changing tsconfig.json like so:

-    "target": "es5",
-    "lib": ["es6", "dom"],
+    "target": "es6",

...and running yarn build && npx http-server build.

I then confirmed the fix works by using my local build of react-scripts-ts:

   "dependencies": {
     "react": "^16.4.2",
     "react-dom": "^16.4.2",
-    "react-scripts-ts": "2.17.0"
+    "react-scripts-ts": "../create-react-app-typescript/packages/react-scripts"

...and running yarn && yarn build && npx http-server build.

Note: this fix increased the boilerplate's un-gzipped JS bundle size from 115969 to 116059 bytes.

[ppvg]

The tests are failing because of SecurityError: localStorage is not available for opaque origins. I haven't looked into why that is. At first glance it seems unrelated. Perhaps someone more familiar with the build system could have a look?

[DorianGrey]

Sorry for the delay - just added a fix to get the CI working again, and restarted this PR's build.
The fix sounds legit, and I'll pick it up once the build is ready.

Side-note: CRA and CRA-TS are using uglifyjs-webpack-plugin, which uses uglify-es. That seems no longer maintained - maybe terser and the corresponding plugin might be favorable; it already includes several fixes, yet I'm not sure if one for the inlining feature is included.

[ppvg]

Thanks!

maybe terser and the corresponding plugin might be favorable

Ah, interesting! Related: facebook/create-react-app#4711. I might look into this if I have some time.