Improvements to demo game

Move widgets and rendering inside src

Improves on demo game balance and adds levels for lasers

Code outside of package:sky should import this code using

package:sky/rendering.dart
package:sky/widgets.dart

Moving this code into the "src" directory is a convention that signifies that
and it cleans up the generated dartdoc because the libraries in the src
directory aren't included in the generated documentation. Instead, the classes
are documented in the widgets.dart and rendering.dart libraries.

Very simple so far. This schedules an alarm to fire once a day, kicking
off a service that downloads a new app.skyx from a hardcoded URL. The
new skyx replaces the current one.

Restore PageableList itemsWrap:true

Makes PageableList with itemsWrap:true work again.

Plumbed the itemsWrap parameter through to HomogenousViewport.

Fixes issue #877.

Fixes issue in sprite constraint

Makes PageableList with itemsWrap:true work again.

Plumbed the itemsWrap parameter through to HomogenousViewport.

Fixes issue #877.

This CL deletes a bunch of unused IDL files and removes some dead code in the
engine.

Still try to sync even if a node has been removed from the tree.

Don't require -p for ios_sim commands

This should handle a case like a stateful component inside a Container
inside another Container having one of those Containers removed while
still keeping that stateful component around with its state.

The problem of how to handle the Container then being reinserted is a
separate issue not handled by this patch.

Add dartdocs for RenderObject

... and other code in object.dart.

Update README.md to reflect removal of --install flag.

Convert Dismissable to use gestures

Convert Dismissable to use the ScrollStart, ScrollUpdate, and ScrollEnd gestures. Support for fling gestures is TBD.

Included a basic unit test that checks that one item can be dismissed with a press-drag-release gesture.

Fixed the scroll gesture recognizer: if the last pointer goes up and candidate recognizers still exist, then reject the gesture.

Convert Dismissable to use the ScrollStart, ScrollUpdate, and ScrollEnd gestures. Support for fling gestures is TBD.

Included a basic unit test that checks that one item can be dismissed with a press-drag-release gesture.

Fixed the scroll gesture recognizer: if the last pointer goes up and candidate recognizers still exist, then reject the gesture.

Positioned 'remembers' things it shouldn't

This patch makes ParentDataNode less general purpose and instead teaches Flex
and Stack how to program the parent data for their children. We used to have
this general system because parent data used to carry CSS styling, but we don't
need it anymore.

Fixes #957

Remove the silly dialog.

Adds a HomogeneousViewport class that works like MixedViewport but
handles only children that have all the same height.

Converts ScrollableWidgetList to use that, so that we don't waste a
frame looking at the size of the contents each time we change size.

This allows a number of seemingly pointless double-pumps in the tests
to be removed.

Other changes that were necessary to support the above:

 - RenderBlock now supports minExtent (think 'min-height' in CSS)
 - RenderBlock now supports itemExtent (forces the height of each
   child to be the same, so that the itemExtent passed to the fixed-
   height scrollables are all authoritative instead of a source of
   bugs when they don't match)
 - RenderBlockViewport now supports horizontal scrolling
 - improved the style of the isInfinite assert in box.dart
 - fixed the position of a comment in mixed_viewport.dart
 - added a test
 - made the logic for how many items to show be more precise