Migration tool has various display issues

[srawlins]

I hope to tackle these simultaneously, so I'm filing this as one bug. Migrate package:collection.

• In lib/src/algorithms.dart, the added required modifier is displayed really weird, making the center panel too wide.
• In lib/src/combined_wrappers/combined_iterable.dart, clicking on an edit makes the right panel too wide, I think because it cannot line-break on the long file name.

[srawlins]

The first item is #40253.