pushBreakContext(breakable, label) {
    this.breakContext = {
      upper: this.breakContext,
      breakable,
      label,
      brokenForkContext: ForkContext.newEmpty(this.forkContext),
    };
    return this.breakContext;
  }
Creates new context for BreakStatement.
@param {boolean} breakable The flag to indicate it can break by
     an unlabeled BreakStatement.
@param {string|null} label The label of this context.
@returns {Object} The new context.