This patch renames StatelessComponent to StatelessWidget and
StatefulComponent to StatefulWidget.

Fixes #2308

Instead of trying to flush the detached children from the child list, we keep
the set of detached children up to date and query on every read.

It has performance implications in release mode.

We'll enable the lint once https://github.com/dart-lang/linter/issues/195 is fixed.

Fixes #2085

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

ScrollableList2 is complete and subsumes all the use cases for the original
ScrollableList.

This patch also changed ScrollableList2 to use an Iterable instead of an
List for its children. This change lets clients map their underlying
data lazily. If the clients actually have a concrete list, we skip the
extra copy and grab the child list directly.

This package contains WidgetTester, which is very useful when writing tests for
widgets.

This seems useful for numbering the entries in the list, for example.

Fixes #328

This patch causes widgets with global keys to drag their state (including their
children) with them as they travel through the element tree.