Truffle/Source: add a new kind of Source (both indexed and non-indexed flavors) whose contents are unavailable at creation, but will be provided incrementally.

function main() {  
  i = 0;  
  while (i < 1000) {
    if (i >= 942) {
      break;
    }  
    i = i + 1;  
  }
  return i;  
}