popChainContext() {
    const context = this.chainContext;

    this.chainContext = context.upper;

    // pop all choice contexts of this.
    for (let i = context.countChoiceContexts; i > 0; --i) {
      this.popChoiceContext();
    }
  }
Pop a `ChainExpression` context from the stack.
This method is called on exiting from each `ChainExpression` node.
This merges all forks of the last optional chaining.
@returns {void}