Show new flutter tool and its use in the widgets dir.

Add a centerSlice parameter to images

Adds publishing for Flutter Sprites

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.

This lets you draw nine-patch images.

Adds check for breaking forces in sprite physics joints

Allow derived classes of Scrollable to listen for scroll events

Add type annotations in many places.
Fix some identifiers to have more lint-satisfying names.
Make all operator==s consistent in style.
Reorder some functions for consistency.
Make ParentData no longer dynamic, and fix all the code around that.

This example is outdated.

Working joints in sprite physics

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

Change the default MaterialType to canvas

We changed IconButton to require a ColorFilter to filter the color of the icon.

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).

We can build Fitness again now that playfair has been updated to account for
the package:flutter rename.

Add a bin/flutter.dart to the flutter package

This command will eventually become the main entrypoint for the Flutter tools.
Also, fixup a reference to the "sky" package in sky_tool.