https://jsfiddle.net/3apqnhmw/

```jsx
class Foo extends React.Component {
  render() {
     return <div>Foo</div>
  }
}

class App extends React.Component {
  render() {
    return Foo
  }
}
```

When `App` returns `Foo` instead of `<Foo />` accidentally, Fiber does not catch this.
returnFiber: Fiber,
    currentFirstChild: Fiber | null,
    newChildren: Array<*>,
    priority: PriorityLevel,
  ): Fiber | null {
    // 
    // 
    // 
    // 
    // 
    // 
    // 
    // 
    // 
    // 
    // 
    // 
    // 
    // 
    // 
    if (__DEV__) {
      // 
      let knownKeys = null;
      for (let i = 0; i < newChildren.length; i++) {
        const child = newChildren[i];
        knownKeys = warnOnInvalidKey(child, knownKeys);