update to CONTRIBUTING.md to use the new update-packages command

improve startup time

flutter analyze: handle missing files

SizeObserver crusade: The Last Test

SizeObserver crusade: ScrollableMixedWidgetListState

Handle lines getting shorter during analysis

SizeObserver crusade: Mimcable

Disabled IconButtons render correctly

Remove AnimationDirection

Fix crash in Block.

Elide fewer AnimationStatus callbacks

This concept is now private to AnimationController. All the clients actually
want the AnimationStatus.

Previously we would elide forward and reverse callbacks that canceled each
other out, which broke the expected state machine. Now we synchronously deliver
status callbacks when start an animation.

Fixes #1913

Stop animating in deactivate()

This assert was overzealous. You can update a RawGestureDetector any
time before the semantics phase, it doesn't have to only be during your
child's layout.

This is good because Block has additional PointerListeners between its
GestureDetector and its Viewport.

SizeObserver crusade: Heroes

Scrollable's callbacks should follow a state machine

remove the isConnected() method from device

Now onScroll callbacks are always bracketed by onScrollStart and onScrollEnd
callbacks.

Fixes #1822

Add tooltips to FloatingActionButtons

SizeObserver crusade: Remove EnterExitTransition

Remove some prints in tests.

Add CheckedPopupMenuitem, update the Stocks demo

Fixes #1875

Somehow this actually made heroes way simpler.

Simplify demo template

We don't need these keys. Also, improve style in several places.

Fixes #2225