**Do you want to request a *feature* or report a *bug*?**
Feature

Right now babel plugin emits globals: https://github.com/facebook/react/issues/16604

```js
window.$RefreshReg$ = () => {};
window.$RefreshSig$ = () => type => type;
```

It would be nice to have them configurable. That would allow to use `import.meta` in environments like SystemJS and have simpler implementation:

```js
import runtime from 'react-refresh/runtime'
runtime.injectIntoGlobalHook(window)

System.constructor.prototype.createContext = function (url) {
  return {
    url,
    $RefreshSig$: runtime.createSignatureFunctionForTransform,
    $RefreshReg$: (type, id) => {
      id = url + ' ' + id
      runtime.register(type, id)
    }
  };
};
```

If you don't mind I could create PR with changes to react-refresh/babel next week.

environment:
```js
{
    "systemjs": "^6.1.4",
    "react": "^16.11.0",
    "react-dom": "^16.11.0",
    "react-refresh": "^0.6.0"
 }
```
seenForRegistration.add(node);
        
        
        
        
        
        
        
        const inferredName = '%default%';
        const programPath = path.parentPath;
        findInnerComponents(
          inferredName,
          declPath,
          (persistentID, targetExpr, targetPath) => {
            if (targetPath === null) {
              
              
              
              
              return;
            }
            const handle = createRegistration(programPath, persistentID);
            targetPath.replaceWith(
              t.assignmentExpression('=', handle, targetExpr),
            );
          },
        );
      },
      FunctionDeclaration: {
        enter(path) {