This property now applies only to the free space in the flex layout (i.e.,
minimize or maximize the amount of free space). Previously, the flexible
children were always allocated a size of zero when MainAxisSize was min. Now
they're allocated the same size that would be if the MainAxisSize was max.

Previously, flexible children were always required to fill their allocated
space. After this change, they can fit loosely into that space and not fill it.
When that happens, the remaining free space is allocated according to the
maixAxisAlignment.

Fixes #5858

Fixes #4618.

Fixes #3289

* justifyContent -> mainAxisAlignment
* alignItems -> crossAxisAlignment
* FlexJustifyContent -> MainAxisAlignment
* FlexAlignItems -> CrossAxisAlignment

Fixes #231

One bug was masking the other, hence they both needed to be fixed
and tested separately.

@Hixie

Add BoxConstraints.isNormalized feature.

Use this feature in asserts in all the intrinsic dimension methods, in
various relevant BoxConstraints methods, and in layout().

Wrap the _DebugSize logic in BoxConstraints.constrain() in an assert
block to avoid a branch in release mode.

Remove the logic in BoxConstraints.isSatisfiedBy() that dealt with
non-normalized values.

Add BoxConstraints.normalize().

Make RenderCustomOneChildLayoutBox.performLayout() only set
parentUsesSize on the child if the constraints aren't tight.

Rather than reading out the maxWidth, we should call constrainWidth to factor
in the minWidth, which might be bigger.