If you change the RenderObject tree between frames, you'll assert if
you subsequently hit test. So e.g. if you get two button presses back
to back, and you mutate the tree synchronously in response to the
first one, the second will assert.

This adds an onBuild callback to WidgetToRenderBoxAdapter to make it
easier to do the updates at the right time, i.e., during widget build.
It'll be called whenever you rebuild the WidgetToRenderBoxAdapter
itself, so all you have to do to use it is call setState() on whoever
is building the WidgetToRenderBoxAdapter.

Previously we used a positional argument for widgets that had multiple
children. Now we use a named argument that defaults to an empty list.

Fixes #241

Instread of an explicit 'enabled' bool, this uses the presence of the
event handler to determine if a widget is enabled or not. This means
that if you've not passed a handler, your widget will be disabled, which
makes sense, since it wouldn't work anyway.

Adds this feature to checkbox, and ports raised button, flat button, and
radio buttons to this new model.

Adds a checkbox to card_collection that can be disabled.

Hide a (basically bogus) hint from the (soon to be disabled) strong hint
mode in the analyzer that this reveals.

Fixes #1669

Also, fix warnings in rendering/sector_layout.dart
Also, fix hit testing in rendering/sector_layout.dart
Also, add WidgetToRenderBoxAdapter
Also, make the rendering library debugging tools more resilient to
dumping stuff before layout is complete.

These examples haven't been ported to fn3 yet. Before this patch, they caused a
lot of noise in the analyzer output.

Canvas is by far the most popular type of material by call site (and makes for
a faster default because it doesn't have a RRect clip).

MaterialApp assumes that you're using material design.

Also move radial reaction and fix imports for stats box.

These are now part of material.dart.

Rather than using a custom render object, we can just use a Stack.

Almost done adding dartdoc to the rendering layer.

This still leaves Flex and FlexDirection available. At some point once
people have transitioned to Row/Column we should rename Flex to _Flex
and stop reexporting FlexDirection from basic.dart.

Fixes #743

Similar to widgets.dart, rendering.dart exports the entire rendering layer.
Also, update the examples to use rendering.dart and widgets.dart. Also clean up
some exports so that the examples have more sensible imports.

It was confusing to have both widget.dart and widgets.dart

Currently we support only borders with uniform color and width.