Right now we use codemirror's JavaScript mode on the website but there's a JSX mode now so we should start using that instead.

lines[i].parent = this;
      height += lines[i].height;
    }
    this.height = height;
  }
  LeafChunk.prototype = {
    chunkSize: function() { return this.lines.length; },
    removeInner: function(at, n) {
      for (var i = at, e = at + n; i < e; ++i) {
        var line = this.lines[i];
        this.height -= line.height;
        cleanUpLine(line);
        signalLater(line, "delete");
      }
      this.lines.splice(at, n);
    },
    collapse: function(lines) {
      lines.splice.apply(lines, [lines.length, 0].concat(this.lines));
    },
    insertInner: function(at, lines, height) {
      this.height += height;
      this.lines = this.lines.slice(0, at).concat(lines).concat(this.lines.slice(at));
      for (var i = 0, e = lines.length; i < e; ++i) lines[i].parent = this;
    },
    iterN: function(at, n, op) {
      for (var e = at + n; at < e; ++at)
        if (op(this.lines[at])) return true;
    }
  };
  function BranchChunk(children) {