This also shrinks the width of the error messages a bit because now that
we use 'package:' URLs the stacks are a bit narrower.

I ran into a case where I was setting minHeight=∞ and then calling
layout() with that constraint, which is all kinds of bad. To try to
catch this earlier, this patch now provides a way to catch constraints
that are requiring infinite values.

We don't _always_ check this because there are valid uses for
BoxConstraints.biggest, e.g. as an additionalConstraint.

Our examples have been growing organically over time. This patch cleans
them up to illustrate specific aspects of Flutter.