ScrollableList2 uses the same pattern as ScrollableGrid, which requires the
client to allocate widgets for every list item but doesn't inflate them unless
they're actually needed for the view. It improves on the original
ScrollableList by not requiring a rebuild of the whole visible portion of the
list when scrolling. In fact, small scrolls can often be handled entirely by
repainting.

Fixes #219

CustomMultiChildLayout delegates to a MultiChildLayoutDelegate. Widget children are specified by their index in the list of children.

Introduces a new Widget that lays its child out, but does not render it.
Also, moves RenderOverflowBox to its own file since it has nothing to do
with proxy box really.

(These are all the debugging-related fixes and trivial typo fixes that I
extracted out of my heroes branch.)

Fix rendering.dart import order.

Introduce a debugLabel for Performances so that when you create a
performance, you can tag it so that if later you print it out, you can
figure out which performance it is.

Allow the progress of a PerformanceView to be determined (but not set).

Allow subclasses of PerformanceView that are constants to be created by
defining a constant constructor for PerformanceView.

Introduce a debugPrint() method that throttles its output. This is a
test to see if it resolves the problems people have been having with
debugDumpRenderTree() et al having their output corrupted on Android. It
turns out (according to some things I read On The Internets) that
Android only has a 64KB kernel buffer for its logs and and if you output
to it too fast, it'll drop data on the floor. If this does in fact
reliably resolve this problem, we should probably move the fix over to
C++ land (where "print" is implemented) so that any use of print is
handled (avoiding the interleaving problem we have now if you use both
debugPrint() and print()).

Fix a bug with the debugging code for "size". In the specific case of a
RenderBox having a parent that doesn't set parentUsesSize, then later
the parent setting parentUsesSize but the child having its layout
short-circuited (e.g. because the constraints didn't change), we didn't
update the _DebugSize object to know that now it's ok that the size be
used by the parent, and we'd assert.

Also, allow a _DebugSize to be used to set the size of yourself.
Previously you could only set your size from a regular Size or from your
child's _DebugSize.

Add more debugging information to various Widgets where it might be
helpful.

Make GlobalKey's toString() include the runtimeType so that when
subclassing it the new class doesn't claim to be a GlobalKey instance.

Include the Widget's key in the Element's description since we don't
include it in the detailed description normally (it's in the name part).

Fix a test that was returning null from a route.

The goal is to follow the guidelines in
https://github.com/flutter/engine/blob/master/sky/specs/style-guide.md#packages

Fixes #1638

MaterialApp assumes that you're using material design.

Also move radial reaction and fix imports for stats box.

Now that you import a top-level file for each layer of the Flutter
framework, within the framework we can use relative paths without being
worried about the copy/pasta problem we used to have.

* Moves from vector_math to vector_math_64
* Adds support for Float64List in Dart bindings

EditableText is now rendered using a custom RenderObject
(RenderEditableParagraph).  RenderEditableParagraph draws the cursor,
handles scroll offsets, and provides feedback about the size of the text for
use by the scroll behavior.

I've also removed the top-level description of the Sky package. Instead, we
should host that content on flutter.io.

If the build stack got long before, it would get cropped.

These files really belong on other libraries.

revert PATH changes

fix typo

some dartdoc changes in files

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.

... and other code in object.dart.

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

@Hixie

Similar to widgets.dart, rendering.dart exports the entire rendering layer.
Also, update the examples to use rendering.dart and widgets.dart. Also clean up
some exports so that the examples have more sensible imports.