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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
staleFamilies.add(family);
      }
    });
    
    const update: RefreshUpdate = {
      updatedFamilies, 
      staleFamilies, 
    };
    helpersByRendererID.forEach(helpers => {
      
      
      helpers.setRefreshHandler(resolveFamily);
    });
    let didError = false;
    let firstError = null;
    
    
    
    
    let failedRootsSnapshot = cloneSet(failedRoots);
    let mountedRootsSnapshot = cloneSet(mountedRoots);
    let helpersByRootSnapshot = cloneMap(helpersByRoot);
    failedRootsSnapshot.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 (!failedRoots.has(root)) {