Refactored backend to remove recursion

[bvaughn]

DevTools doesn't currently handle extremely deep trees (facebook/react#16491) or extremely wide trees (facebook/react#16501) very well, due to recursion in the backend interface. This PR removes most of that.

As a sanity test, I confirmed that after this refactor, DevTools was able to handle the following:

const Child = ({ children = null }) => children;

const Deep = () => {
  let children = null;
  for (let i = 0; i < 15000; i++) {
    children = <Child>{children}</Child>;
  }
  return children;
};

const Wide = () => {
  let children = [];
  for (let i = 0; i < 15000; i++) {
    children.push(<Child key={i} />);
  }
  return children;
};

[gaearon]

Why is it not expected?

[bvaughn]

Maybe I misread the function? 🙂 I'm not as familiar with the suspense logic since I didn't write it. I just skimmed it and read the header comment:

// Returns whether closest unfiltered fiber parent needs to reset its child list.

It seemed like it didn't recurse as greedily as the mount/unmount functions. Looking at our tests it also seemed like the function didn't recurse as deeply as the others (primarily mount) but that could just be due to it being a small sample size.

Think we should rewrite it also? I would have just done it initially, except that it seemed trickier than the other two since it also depends on its own return value.

[kaykayehnn]

It seems that traversing deep trees causes this function to stack overflow. See this example

const Child = ({ children = null }) => children;

const Deep = () => {
  const [condition, setCondition] = useState(false);

  let children = null;
  for (let i = 0; i < 15000; i++) {
    children = <Child>{children}</Child>;
  }
  return (
    <div>
      <button onClick={() => setCondition(c => !c)}>Rerender</button>
      {children}
    </div>
  );
};

The initial render works, but rerendering the component recurses too deeply and overflows. Adding the same rerender button on the Wide component works fine though.

[bvaughn]

Going to migrate this PR to the React repo -> facebook/react#16627