function parseMaybeAssign(noIn) {
    var left = parseMaybeConditional(noIn);
    if (tokType.isAssign) {
      var node = startNodeFrom(left);
      node.operator = tokVal;
      node.left = left;
      next();
      node.right = parseMaybeAssign(noIn);
      checkLVal(left);
      return finishNode(node, "AssignmentExpression");
    }
    return left;
  }
Parse an assignment expression. This includes applications of
operators like `+=`.

@param {boolean|undefined} noIn
@returns {!node_t}