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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
const [val, setVal] = React.useState(0);
          return (
            <p style={{color: 'blue'}} onClick={() => setVal(val + 1)}>
              {val}
            </p>
          );
        }
        $RefreshReg$(Hello, 'Hello');
        return Hello;
      });
      
      const el = container.firstChild;
      expect(el.textContent).toBe('0');
      expect(el.style.color).toBe('blue');
      act(() => {
        el.dispatchEvent(new MouseEvent('click', {bubbles: true}));
      });
      expect(el.textContent).toBe('1');
      
      let HelloV2 = patch(() => {
        class Hello extends React.Component {
          state = {count: 0};
          handleClick = () => {
            this.setState(prev => ({
              count: prev.count + 1,
            }));
          };
          render() {