**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"
 }
```
const B = foo ? require('X') : require('Y');
        const C = requireCond(gk, 'C');
        const D = import('D');
        export default function App() {
          return (
            <div>
              <A />
              <B />
              <C />
              <D />
            </div>
          );
        }
    `),
    ).toMatchSnapshot();
  });
  it('can handle implicit arrow returns', () => {
    expect(
      transform(`
        export default () => useContext(X);
        export const Foo = () => useContext(X);
        module.exports = () => useContext(X);
        const Bar = () => useContext(X);
        const Baz = memo(() => useContext(X));
        const Qux = () => (0, useContext(X));
      `),
    ).toMatchSnapshot();
  });
});