Fix scrolling of Block.

Since our build function depends on scrollBehavior.isScrollable, any
time we update scrollBehavior we are implicitly updating our state. As
such, we must do so during a setState() call, or else we won't rebuild
and might not bother to listen to the scroll gestures.

This probably broke when we made Block not listen to gestures if it
wasn't overflowing.

There are some TODOs in this file that I'll address in the next patch.

More dartdocs for fn3, refactor buildDirtyElements

Ensure that EditableText always shows a cursor

Some cleanup of the Sky analyzer script

The buildDirtyElements() method now enforces that we only build deeper
nodes and never go back to build higher nodes during a frame.

- move _uniqueChild earlier since a comment now mentions it earlier.
- reorder methods in Element to more closely reflect call order.
- change mount to be the place that sets the parent pointer, for consistency.
- make the lifecycleState a purely debug-time thing for consistency.
- make BuildableElement.unmount set dirty to false so that we won't
  build unmounted objects.
- rename "updated" to "newWidget" for clarity.
- change how we unmount things so that the slot is reset up to a
  RenderObjectElement, but not further, and don't reset the depth.

All the use cases for EventDisposition are now addressed by the gesture
detection system.

We no longer have gesture events.

Add a LeafRenderObjectElement, fix a test to use the new contract.

- adds dartdocs
- replaces config setter with didUpdateConfig() so that you can't replace
the config outside of the system
- renames didUnmount() with destroy().
- rename Component to StatelessComponent, ComponentConfiguration to
StatefulComponent
- move debug dump to end of file
- renamed _holder to _element

fn3 shouldn't rebuild components that don't change identity

add a test for scale gestures

* If no source path is provided, then run the analyzer on the Sky unit tests
* Fix the filter for errors found in pub cache packages
* Generalize the filter for the analyzer's "xx errors/warnings/hints found" status message

Also fix a test that caused a warning in the analyzer.

Previously EditableText would render a text widget with no cursor if the text
value was empty.

Also adjust the height of the cursor widget to reflect the style's line
height, and update the cursor painting to match.

Previously --stage-two would error out because the LICENSE was git ignored.
This patch includes the LICENSE file in the repo to avoid the error. Also,
refactor the big_red_button.py to be clearer about what's shared between stage
one and stage two of the release process.

Prototype of fn3

We were just missing a few type casts in tests to make the analyzer happy.

This patch contains a prototype of a new widget framework. In this framework,
Components can be reused in the tree as many times as the author desires. Also,
StatefulComponent is split into two pieces, a ComponentConfiguration and a
ComponentState. The ComponentConfiguration is created by the author and can be
reused as many times as desired. When mounted into the tree, the
ComponentConfiguration creates a ComponentState to hold the state for the
component. The state remains in the tree and cannot be reused.

Instead of having to manage the popup menu from your app's build
function, you now just call showPopupMenu() with the menu's position and
it takes care of everything for you.

This solves the problem that the popup menu was trying to mutate the
state of the navigator from within its own initState() function.

Also, remove the "route" argument to RouteBase.build() since it equals
"this" by definition...

Also, remove ModalOverlay, and instead put that logic in the navigator.

Improvements to EffectLine in sprites

Move findAncestorRenderObjectWrapper to RenderObjectWrapper

Fix Rect intersection; add test

That way it's closer to where it's used, and it's more obvious that
anyone can write such functions.

- Fix AnimationTiming to have defaults for 'interval' and 'curve' since
  that seems to be how we use it.

- Merge RouteBase.build and RouteBase.buildTransition

- Get rid of HistoryEntry, since it added nothing

- Broke out RouteBase.createPerformance() so subclasses can change what
  is created.

- Build the routes backwards so that we more efficiently avoid building
  hidden routes.

- Introduce an explicit way (!hasContent) for RouteState to avoid
  building, rather than the implicit "build returns null" we had before.

In EditableText, wrap the placeholder text in a Row so it will expand to its parent's width

This will ensure that the width of an empty Input is consistent with the
width of an Input that contains text.

Also add a unit test for the Input widget and a way for tests to provide mock
implementations of Mojo services such as the keyboard.

SkySprites should import the public libraries

Require that you pass transitions a performance.