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.)

Also, rename build_utils.dart to widget_tester.dart. These files are now named
for their most commonly used classes.

Finally, add a .analysis_options to silence the (intentional) analyzer warnings
in append_child_test.dart.

Merge pumpPaintFrame and pumpFrame in WidgetTester

We can make all the test pass by removing more of the mocks and using the real
code.

Test year selection for date picker

Fix ProgressIndicators and add a regression test

Added some unit test infrastucture for checking layers.

Added some unit test infrasture for checking layers.

Generalise RenderBlockViewport so that it can be used by a Widget that knows its children's dimensions.

- add debugDescribeSettings to a few classes that were missing it
- fix some minor bugs in RenderShrinkWrapWidth and ShrinkWrapWidth
- introduce RenderShrinkWrapHeight and ShrinkWrapHeight

Generalise RenderBlockViewport so that it can be used by a Widget that knows its children's dimensions.

I'll write the Widget wrapper in the next CL, including adding
support for padding at the Widget layer.

@Hixie

Make RenderBlockViewport shrink-wrap its children in the main axis direction

Add a basic test for DatePicker

Most of this work in this patch is building out the test framework to the point
where we can write this test.

Update the documentation on sizing, make another assert point to it.

Moves GameObjectFactory and PlayerState to their own files in demo game

Fixes #454.

Adds power ups to demo game

Turn big_circle.dart into a test

Fixes #829

Sector demo is broken because of compositor changes

We need to implement visitChildren for the RenderObjects in the sector demo.
Also, add a test.

Fixes #790

Fixes issues in sprite constraints