<!--
  Note: if the issue is about documentation or the website, please file it at:
  https://github.com/reactjs/reactjs.org/issues/new
-->

**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 instancesForRoot = helpers.findHostInstancesForRefresh(
        root,
        families,
      );
      instancesForRoot.forEach(inst => {
        affectedInstances.add(inst);
      });
    });
    return affectedInstances;
  } else {
    throw new Error(
      'Unexpected call to React Refresh in a production environment.',
    );
  }
}
export function injectIntoGlobalHook(globalObject: any): void {
  if (__DEV__) {
    
    
    
    
    let hook = globalObject.__REACT_DEVTOOLS_GLOBAL_HOOK__;
    if (hook === undefined) {
      
      
      
      let nextID = 0;
      globalObject.__REACT_DEVTOOLS_GLOBAL_HOOK__ = hook = {
        supportsFiber: true,
        inject(injected) {