Rather than requiring the developer to specify a full Decoration, we now
just take an ImageProvider for the thumb image. Also, fix
ImageFit.scaleDown to actually work.

Fixes #4571
Fixes #4673

We now use the `@required` annotation to encourage developers to
explicitly set onPressed and onChanged callbacks to null when that would
disable the widget.

Fixes #287

Use the route table to generate the list of screens to test in the smoke test.

* Gallery code snippets now analyzed

See https://www.google.com/design/spec/style/writing.html#

These are called selection controls in the spec and in the gallery UI.

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

Fixes #231

* left -> leading (Removes an LTR bias)
* center -> title (Widget was actually centered)
* right -> actions (Removes an LTR bias, asymmetric with leading)

Fixes #2348

This patch renames StatelessComponent to StatelessWidget and
StatefulComponent to StatefulWidget.

Fixes #2308