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.

this.children = children;
    var size = 0, height = 0;
    for (var i = 0, e = children.length; i < e; ++i) {
      var ch = children[i];
      size += ch.chunkSize(); height += ch.height;
      ch.parent = this;
    }
    this.size = size;
    this.height = height;
    this.parent = null;
  }
  BranchChunk.prototype = {
    chunkSize: function() { return this.size; },
    removeInner: function(at, n) {
      this.size -= n;
      for (var i = 0; i < this.children.length; ++i) {
        var child = this.children[i], sz = child.chunkSize();
        if (at < sz) {
          var rm = Math.min(n, sz - at), oldHeight = child.height;
          child.removeInner(at, rm);
          this.height -= oldHeight - child.height;
          if (sz == rm) { this.children.splice(i--, 1); child.parent = null; }
          if ((n -= rm) == 0) break;
          at = 0;
        } else at -= sz;
      }
      if (this.size - n < 25) {
        var lines = [];