dart HTML TableCellElement - cannot get or set colspan
Issue https://code.google.com/p/dart/issues/detail?id=15256 was created.
What you are seeing are hints, not warnings, out of the analyzer. (There is a bug to fix the UI to make this more clear).
If you change var table to TableElement table, then the analyzer will violate clospan as you probably expected. Since you declared table as a "var" the analyzer doesn't produce a warning. Since this can lead to bugs like the one you encountered, we are always in the process of adding hints.
Unfortunately, there seems to be a bug where the propagated type of table.rows.last.cells.last is not being resolved. I am looking into this now.