src/doc/doc-builders.js/ifBreak
function ifBreak(breakContents, flatContents) {
  if (process.env.NODE_ENV !== "production") {
    if (breakContents) {
      assertDoc(breakContents);
    }
    if (flatContents) {
      assertDoc(flatContents);
    }
  }

  return { type: "if-break", breakContents, flatContents };
}