https://codesandbox.io/s/40j50607y0

Rendering a component like:

```js
const Foo = () => <React.Fragment />
```

Throws the error:

> Foo(...): Nothing was returned from render. This usually means a return statement is missing. Or, to render nothing, return null.

The error is confusing because clearly `Foo` _is_ returning a value. I don't know if we want to support rendering empty fragments, but in any case this error is confusing.

If this isn't allowed it should explicitly call out that a component cannot return a fragment with no children.

cc @gaearon 
const instance = returnFiber.stateNode;
            if (instance.render._isMockFunction) {
              // 
              break;
            }
          }
        }
        // 
        // 
        // 
        case FunctionalComponent: {
          const Component = returnFiber.type;
          invariant(
            false,
            '%s(...): Nothing was returned from render. This usually means a ' +
              'return statement is missing. Or, to render nothing, ' +
              'return null.',
            Component.displayName || Component.name || 'Component',
          );
        }
      }
    }
    // 
    return deleteRemainingChildren(returnFiber, currentFirstChild);
  }
  return reconcileChildFibers;
}
export const reconcileChildFibers = ChildReconciler(true);
export const mountChildFibers = ChildReconciler(false);
export function cloneChildFibers(