The asserting will continue until morale improves!

* Convert all assert(*.isNormalized) checks to use the new
  debugAssertIsNormalized technology.
* Convert CustomMultiChildLayout to use the new RenderingError
  technology to greatly improve the detail in errors you get when
  writing CustomMultiChildLayout delegates.
* Add BoxConstraints.copyWith().
* Indent the descendants in the rendering exception data dump so that
  when you have multiple children it's clearer what's going on.

There aren't any clients anymore.

Fixes #1684

Introduces a new Tooltip class.
Adds support for tooltips to IconButton and Scaffold.
Adds some tooltips to various demos.

Also some tweaks to stack.dart that I made before I decided not to go
down a "CustomPositioned" route.

Previously, we triggered a layout (and hence a repaint) when sliding the
draw because we gave the draw loose constraints. The drawer uses an
Align to move itself to the proper side of the screen, so it can have
tight constraints, which makes it a layout boundary.

Also, don't trigger a layout just because the Scaffold rebuilds. There
isn't any state in the scaffold custom layout, so it doesn't need to
repaint just because we created a new instance of the delegate.

Finally, add the debugging infrastructure I used to find these issues.

Previously we used Position, which makes it harder to accumulate offsets
when walking the render tree.

CustomMultiChildLayout and CustomOneChildLayout now use their delegate's shouldRelayout() method instead of a "token" to decide if layout is needed.

MultiChildLayoutDelegate and OnChildLayoutDelegate are now expected to be stateless, i.e. they'll typically be built each time their custom layout widget is built. If the identical layout delegate is provided to a new custom layout, layout will not happen.

Revised the bottom sheet implementation per the new custom layout classes. Removed a SizeObserver.

Fixes #899

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.

These docs are largely duplicated from the cooresponding rendering classes. I'm
not sure how we can avoid repeating ourselves.

Add some missing functionality to ParentData subclasses.

Fixes #474

Fixes #692
Fixes #690

Prior to this patch, MultiChildLayoutDelegate couldn't be re-entered because it
cleared _idToChild when unwinding its stack. Now we restore the previous value
of _idToChild when we unwind.

Now a RenderBox is considered hit if one of its children are hit or it itself
decides that it's hit. In particular, empty space inside a flex won't be hit
because none of the children are located there and a RenderFlex doesn't
consider itself hittable.

Fixes #53
Fixes #1221

CustomMultiChildLayout delegates to a MultiChildLayoutDelegate. Widget children are specified by their index in the list of children.