Animated sprite physics bodies now correctly transfers energy to dynamic bodies

The goal is to follow the guidelines in
https://github.com/flutter/engine/blob/master/sky/specs/style-guide.md#packages

Fixes #1638

Export new InputEvent classes from appropriate places

It's OK to just specify the colors list.

Fixes SpriteBox to work better with native points

Improves sprite physics debug drawing

Arena sweep

Fix warnings seen when running skyanalyzer in strong mode

Teach dumpRenderTree() to draw actual trees.
Make the TextStyle output terser so it doesn't overflow the output.
Make debugDumpApp() say what mode we're in (checked vs release).
Hide lifecycle state from release mode dumps (since it's checked-only state).

Fixes touch handling and refactors sprite physics

We had a typo in our CSS translation.

Implement better synchronization between the state of the input widget and the keyboard IME

Add a ListItem widget

For those times where you want to spin something around a point relative
to the size of your box, but you don't know the size of your box.

Also add an input widget API for the keyboard submit button

Add dart-layer input event abstraction

Adds missing sprite physics shapes

Update to mojo 1292152bf533de1bfebf5966d6246e149f87add3

Enabled displaying a scrollbar in ScrollingLists. The scrollbar is painted as an "overlay", i.e. it's painted on top of the scrolling list's visible children.

Added an abstract Painter base class that encapsulates a paint method and the renderer that it's attached to. RenderBlockViewport and HomogenousViewport now support an overlayPainter property. If specified, RenderBlockViewport attaches itself to the overlayPainter when it's attached to the rendering tree. RenderBlockViewport now calls overlayPainter.paint() after it has painted its children.

Added an abstract ScrollingListPainter class that exposes ScrollingList's state which might be needed for painting. Like its scroll direction and scrollOffset. The ScrollingListPainter is notified when a scroll starts and ends.

Defined a Material-specific ScrollingListPainter that renders a scrollbar. The scrollbar thumb is faded in/out when the scroll starts/ends.

Added onScrollStart and onScrollEnd listeners to Scrollable.

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