Repository: create-react-app
Issue: Non-dead JavaScript is removed during build
Labels: issue: bug

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### Describe the bug

`react-scripts build` seems to be removing inline JavaScript code from index.html that should not be removed. It happens to code paths that are conditional and the condition contains a build-time '%VARIABLE%'.

Workaround: move the build-time variable outside of the condition. 

### Did you try recovering your dependencies?

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No, it's a newly created project.

### Which terms did you search for in User Guide?

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NODE_ENV, index.html, variables

### Environment

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```
  System:
    OS: macOS 10.14.5
    CPU: (4) x64 Intel(R) Core(TM) i5-5287U CPU @ 2.90GHz
  Binaries:
    Node: 8.16.0 - /usr/local/bin/node
    Yarn: 1.17.3 - /usr/local/bin/yarn
    npm: 6.10.1-salomvary1 - /usr/local/bin/npm
  Browsers:
    Chrome: 75.0.3770.142
    Firefox: 68.0.1
    Safari: 12.1.1
  npmPackages:
    react: ^16.8.6 => 16.8.6 
    react-dom: ^16.8.6 => 16.8.6 
    react-scripts: 3.0.1 => 3.0.1 
  npmGlobalPackages:
    create-react-app: 3.0.1
```

### Steps to reproduce

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1. Create a new app with `create-react-app`.
2. Edit `public/index.html` and add a the snipped below after `<body>`.
3. Run `npm run build`.
4. Open `build/index.html`

```
    <script>
      console.log('%NODE_ENV%');
      var nodeEnv = '%NODE_ENV%';
      if ('%NODE_ENV%' === 'production') {
        console.log('This is production!');
      }
      if (nodeEnv === 'production') {
        console.log('This is REALLY production!');
      }
    </script>
```

### Expected behavior

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`build/index.html` should contain the minified version of the entire snippet with `%NODE_ENV%` replaced with `production`.

### Actual behavior

This is what `build/index.html` contains instead (note that the first if block is entirely missing):

```
<script type="text/javascript">
console.log("production");var nodeEnv="production";"production"===nodeEnv&&console.log("This is REALLY production!")
</script>
```

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### Reproducible demo

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https://github.com/salomvary/create-react-app-bug

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