rev the engine

Minor fixes in comments and style.

Upgrade the test package (needed by the latest mojo roll)

applyPaintTransform() improvements

Previously, applyPaintTransform() had to know how it was positioned in
its parent, even though that's really the parent's responsibility.

Now, applyPaintTransform() is given a child and applies the transform
that it would give the child during paint.

This makes it possible for applyPaintTransform() to work across
coordinate system boundaries (e.g. box to sector, or view to box --
previously, view to box only worked because we explicitly skipped that
step -- since view doesn't actually apply a transform, it doesn't
really matter).

Check for cycles in our various tree structures.

Add dartdoc for image classes

 - Adds dartdoc for all the `of` functions.
 - Renames Image to RawImage. This widget is rarely used and shouldn't take up
   such a nice global name.

Fixes #361

Remove reference to scheduler in animation.

Add more dartdoc for basic widgets

Added rotation test for new PageableList code.

TabBarView is-a PageableList, doesn't need itemExtent

Base TabBarView on PageableList so that itemExtent and its SizeObserver aren't needed.

TabBarView scrolling is still TBD.

Improve asserts when performLayout missing

Fixes #566

Improve error messages when build fails

Recognize offline status in adb

Instead of failing with a cryptic error message about app.flx, we now fail with
an explicit message about the compiler.

Releated to #865

Remove debugDoingPaint setter.

Fixes #858

Add more dartdoc

debugDoingPaint is an internal flag and really shouldn't be globally settable.

Support an empty PageableList

Corrected support for PagebleList when zero items or no items are specified.

Added some final qualifiers in pageable_list.dart

These docs cover some undocumented enums and some of the basic layout widgets.

Minor modifs.

Remove last traces of onEvent.

Fix PageableList demo

PageableList's scrollBehavior depends on its itemsWrap property.

Also: deleted some commented out code that I'd forgotten to delete.