Fix SplitPane RangeError when child count changes between rebuilds#9822

kenzieschmoll merged 4 commits intoflutter:masterflutter/devtools:masterfrom

ishaquehassan:fix/9648-splitpane-rangeerror-on-children-count-changeishaquehassan/devtools:fix/9648-splitpane-rangeerror-on-children-count-changeCopy head branch name to clipboard