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

CustomMultiChildLayout and CustomOneChildLayout now use their delegate's shouldRelayout() method instead of a "token" to decide if layout is needed.

MultiChildLayoutDelegate and OnChildLayoutDelegate are now expected to be stateless, i.e. they'll typically be built each time their custom layout widget is built. If the identical layout delegate is provided to a new custom layout, layout will not happen.

Revised the bottom sheet implementation per the new custom layout classes. Removed a SizeObserver.

Fixes #899

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

- force the time dilation to 1.0 for the Widget tests, so that a local
  change doesn't break all the tests during development.
- add missing license block to all the files.
- set ui.window.onBeginFrame to null when you use WidgetTester, so that
  the engine doesn't trigger any confusing frames after our fake frames.

Prior to this patch, MultiChildLayoutDelegate couldn't be re-entered because it
cleared _idToChild when unwinding its stack. Now we restore the previous value
of _idToChild when we unwind.

CustomMultiChildLayout delegates to a MultiChildLayoutDelegate. Widget children are specified by their index in the list of children.