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**Do you want to request a *feature* or report a *bug*?**

feature

**What is the current behavior?**

When `react-dom` is loaded from CDN like `<script src="https://cdn.jsdelivr.net/npm/react-dom@16.12.0/umd/react-dom.development.js"></script>`, `react-refresh` failed to inject hook into devtools.

I've created a related issue here: https://github.com/pmmmwh/react-refresh-webpack-plugin/issues/13

We should find a way to invoke `injectIntoGlobalHook` function from `react-refresh/runtime`, however this file is in cjs format so we cannot currently do this in a simple way.

**What is the expected behavior?**

I'd like `react-refresh` to publish runtime as a umd bundle so we can reference it from CDN and put it before `react-dom`'s `<script>` element, then invoke `injectIntoGlobalHook` in the right place.

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**

- `react-dom`: 16.12.0
- `react-refresh`: 0.7.0
try {
      const toString = Function.prototype.toString;
      const code = toString.call(fn);
      
      
      
      if (code.indexOf('^_^') > -1) {
        
        hasDetectedBadDCE = true;
        
        
        setTimeout(function() {
          throw new Error(
            'React is running in production mode, but dead code ' +
              'elimination has not been applied. Read how to correctly ' +
              'configure React for production: ' +
              'https:fb.me/react-perf-use-the-production-build',
          );
        });
      }
    } catch (err) {}
  }
  let uidCounter = 0;
  function inject(renderer) {