Add a build step to hoist warning conditions

[gaearon]

I was recently profiling a DEV build of React and noticed this:

So almost 10% of the time was spent building component stacks for a warning that doesn't fire once. This is because the code looks like

warning(condition, message, getStack())

and so we evaluate getStack() regardless of whether it fires.

We already have a build step that "hoists" invariant conditions to prevent unnecessary work for invariants. In this PR, I extend an existing warning build step to do the same thing.

Essentially it turns the above code into

if (!condition) {
  warning(false, message, getStack());
}

This is nice both because it delays evaluating the arguments for the common case, and because we don't even have to step into warning at all.

I enabled it in tests too for parity.

One potential pitfall is that it's easier to introduce a mistake into the argument calculation, and if it's never evaluated in tests, we'll never know. But we generally don't add warnings without tests so that seems like it won't be a big problem in practice.

Full bundle diff: https://gist.github.com/gaearon/0bc2fd23d957e2ddc3939ef7425b3afd

This has no effect on production bundles because they don't contain warning calls.

[bvaughn]

Looks like a good change to me.

[bvaughn]

Seems kind of silly that we're using fbjs for warning and invariant at this point 😁

[gaearon]

I think we still need for React Native (and to go through our custom fbsource RN infra there).
Although we might as well have implemented that through the new forking infra.

[bvaughn]

Right, that's what I meant. We may have well just built our own thing.