sS jsonEncode_breakAtLevels(int levels default 5, O o) {
  new StringBuilder buf;
  jsonEncode_breakAtLevels(levels, o, buf, 1);
  ret str(buf);
}

static void jsonEncode_breakAtLevels(int levels, O o, StringBuilder buf, int level) {
  if (o == null)
    buf.append("null");
  else if (o instanceof S)
    buf.append(quote((S) o));
  else if (o instanceof Number || o instanceof Bool)
    buf.append(o);
  else if (o instanceof Map) {
    Map map = cast o;
    buf.append("{");
    bool first = true;
    for (O key : keys(map)) {
      if (first) first = false; else buf.append(",");
      if (level == 1) buf.append("\n" + spaces(level*2));
      buf.append(quote((S) key));
      buf.append(":");
      jsonEncode_breakAtLevels(levels, map.get(key), buf, level+1);
    }
    if (level <= levels) buf.append("\n" + spaces((level-1)*2));
    buf.append("}");
  } else if (o instanceof Collection) {
    Collection l = cast o;
    buf.append("[");
    bool first = true;
    for (O element : l) {
      if (first) first = false; else buf.append(",");
      if (level <= levels) buf.append("\n" + spaces(level*2));
      jsonEncode_breakAtLevels(levels, element, buf, level+1);
    }
    if (level <= levels) buf.append("\n" + spaces((level-1)*2));
    buf.append("]");
  } else
    fail("Unknown object for JSON encoding: " + className(o));
}

Began life as a copy of #1012984