Could one of TV’s most enduring, and endearing, sitcoms depart with the death of a significant character? That’s the speculation on social media after last week’s pivotal episode of “How I Met Your Mother,” which airs its series finale March 31. Many fans are outraged at even the suggestion of anything less than a sunshiny ending, but some of history’s most memorable sitcoms were determined to make us sniffle between laughs. In preparation for Monday’s new episode of “Mother,” we reflected on 10 sobering season finales.