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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
'Unexpected call to React Refresh in a production environment.',
    );
  }
}
export function hasUnrecoverableErrors() {
  
  return false;
}
 Exposed for testing.
export function _getMountedRootCount() {
  if (__DEV__) {
    return mountedRoots.size;
  } else {
    throw new Error(
      'Unexpected call to React Refresh in a production environment.',
    );
  }
}
export function createSignatureFunctionForTransform() {
  if (__DEV__) {
    let call = 0;
    let savedType;
    let hasCustomHooks;
    return function<T>(
      type: T,
      key: string,
      forceReset?: boolean,
      getCustomHooks?: () => Array<Function>,
    ): T {
      switch (call++) {