Add scroll gestures and use them in Scrollable

This patch replaces the scroll gestures used by Scrollable with ones detected
by the ScrollGestureDetector.

Add a way of having keys based on numeric types or DateTimes by having a ValueKey<T> class.
Remove the redundant ways of declaring things, except for leaving one shorthand -- you can say `new Key(s)` instead of `new ValueKey<String>(s)`.

Stop exporting framework.dart from basic.dart, since we now have widgets.dart exporting all of framework.dart.

Use GestureDetector in the framework and examples

Add a --verbose flag to sky_tool

Also clean up some formatting issues.

We're now using it at the widget layer for everything except scrolling and
flinging.

Add tap, show press, and long press gestures and use tap in IconButton

These gestures use the new gesture detection system.

Add --release flag

Add GestureArena

In the GestureArena, gestures will fight it out to see which is the real
ultimate gesture.

Add basic Grid widget and test RenderGrid

Allow parents of RenderObjects that have layout callbacks to depend on those objects' sizes.

Fix infinite loop

Previously, if you used a layout callback, you could not have a parent
that did parentUsesSize, or if you did, you had to be marked
sizedByParent. With this patch, we allow the parent to depend on your
layout, even if you modify your child list during your layout using a
layout callback, by checking that the parent is still actively being
laid out in this scenario.

Add PointerRouter

Only require iOS 7 for Fitness app

This patch is the first step towards implementing gestures. The pointer router
allows the gesture detectors to hook in at the end of the pointer event
propagation chain.

Related to #145

@abarth

Also includes changes to widget_tester sufficient to
test scrolling.

@abarth

Scheduler should be able to cancel a queued callback

Start "sky_tool listen" with a refresh

Have “sky_tool listen” start with a refresh so that iOS devices don’t wait on a black screen for the first filesystem change.

Previously, once a callback was in the execution queue, it couldn't be
canceled. Now we check whether the callback was canceled before executing it.

Abstract out syncChildren().

Adds counting of coins in demo game

Add logging to help diagnose image loading failures

For debugging #801

The core of MultiChildRenderObjectWrapper.syncRenderObject() could
apply to any subclass that uses a flat child list, so this abstracts
it out into the superclass.

(Also, instead of requiring the callbacks of RenderBlockViewport to
constrain their results, we just constrain it for them. Makes things a
bit easier for users of that class.)