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.

Use explicit service name for activity service

Update to the new TextDecoration API

Now with fewer lists.

Let clients of Checkbox, Radio, Slider, and Switch customize the activeColor

We might want to let folks customize the inactive color too, but customizing
the active color is a good place to start.

Revised PageableList et al

An itemExtent-computing SizeObserver is no longer needed to use PageableList. The PageableList just uses its own size as the itemExtent.

Added the itemsSnapAlignment property to PageableList which enables snapping scrolls to an adjacent item (the default), or any item boundary no not at all.

PageableList scrollOffsets now vary from 0.0 to itemCount instead of 0.0 to itemExtent * itemCount. Using logical coordinates instead of pixel coordinates means that the scroll position is insensitive to changes in the PageablList's size.

Added HomogenousPageViewport which is used by PageableList. HomogenousPageViewport scrollOffsets are defined as for PageableList.

Factored the (substantial) common parts of HomogenousViewport HomogenousPageViewport into a file private _ViewportBase class.

Removed PageableWidgetList. PageableList now just extends Scrollable. Moved PageableList into its own file.

Removed the pixel dependencies from ScrollBehavior. ScrollBehavior.createFlingSimulation() no longer sets the simulation's tolerance. The caller must do this instead.

Scrollable now uses pixelToScrollOffset() to convert from input gesture positions and velocities to scrollOffsets.

Fixes #710