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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
return <h1>Nice.</h1>;
        }
        $RefreshReg$(Hello, 'Hello');
      });
      expect(container.firstChild.nextSibling).toBe(helloNode);
      expect(helloNode.textContent).toBe('Nice.');
    }
  });
  it('remounts error boundaries that failed asynchronously after hot update', () => {
    if (__DEV__) {
      render(() => {
        function Hello() {
          const [x] = React.useState('');
          React.useEffect(() => {}, []);
          x.slice(); 
          return <h1>Hi</h1>;
        }
        $RefreshReg$(Hello, 'Hello');
        class Boundary extends React.Component {
          state = {error: null};
          static getDerivedStateFromError(error) {