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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
}
      if (rootElements === null) {
        return;
      }
      if (!rootElements.has(root)) {
        return;
      }
      const element = rootElements.get(root);
      try {
        helpers.scheduleRoot(root, element);
      } catch (err) {
        if (!didError) {
          didError = true;
          firstError = err;
        }
        
      }
    });
    mountedRootsSnapshot.forEach(root => {
      const helpers = helpersByRootSnapshot.get(root);
      if (helpers === undefined) {
        throw new Error(
          'Could not find helpers for a root. This is a bug in React Refresh.',
        );
      }
      if (!mountedRoots.has(root)) {
        
      }
      try {