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.

New RenderAligningShiftedBox abstract class for render boxes with
"alignment", which as of now is:

 - RenderPositionedBox
 - RenderConstrainedOverflowBox (ex RenderOverflowBox)
 - RenderSizedOverflowBox
 - RenderFractionallySizedOverflowBox (ex RenderFractionallySizedBox)

(The respective widgets are Align, OverflowBox, SizedOverflowBox, and
FractionallySizedBox. They haven't been renamed.)

Also I added some toString details.

Fixes https://github.com/flutter/flutter/issues/2735