* Don't show "context" for classes without either contextType or contextTypes
* For classes with contextTypes, change the label to "legacy context" instead

---
Originally reported by @gaearon via https://github.com/bvaughn/react-devtools-experimental/issues/368
I
     u
    if (a.tag !== HostRoot) {
      throw Error('Unable to find node on an unmounted component.');
    }
    if (a.stateNode.current === a) {
       W
      return fiber;
    }
     O
    return alternate;
  }
   E
  function prepareViewElementSource(id: number): void {
    let fiber = idToFiberMap.get(id);
    if (fiber == null) {
      console.warn(`Could not find Fiber with id "${id}"`);
      return;
    }
    const {elementType, tag, type} = fiber;
    switch (tag) {
      case ClassComponent:
      case IncompleteClassComponent:
      case IndeterminateComponent:
      case FunctionComponent:
        global.$type = type;
        break;
      case ForwardRef:
        global.$type = type.render;
        break;