Add a AnimatedRelativeRectValue class for animating RelativeRects.

Add a PositionedTransition class for animating Positioned using
AnimatedRelativeRectValues.

Add a test for PositionedTransition.

Fix a math bug a RelativeRect found by the test.

Fix a logic bug in the two ParentDataWidget classes found by the test.
Specifically, they were marking the child dirty, rather than the parent.
The parentData is for the parent's layout, not the child's, so they have
to mark the parent dirty. (I didn't hoist this up to the superclass
because ParentData could be used for painting, hit testing,
accessibility, or any number of other things, and I didn't want to bake
in the assumption that it needed markNeedsLayout.)

- truncate pixel values to 1dp since there's really no point being told
  the Size is 302.98732587287 by 648.28498579187.

- describe more Widgets so that debugDumpApp() is more useful.

- remove bufferValue from ProgressIndicator (cc @hansmuller) since it's
  not yet implemented.

- half-hearted toString() for ThemeData. There's no point making a
  complete one, since it would cause line-wrap even on big monitors in
  debugDumpApp dumps, and you can easily get the actual values from a
  debugging if that's the issue.

- flesh out BoxConstraints.toString() so that fully unconstrained and
  fully infinite constraints are called out explicitly. I experimented
  with adding even more special cases, e.g. calling out unconstrained
  widths with fixed heights, etc, but it made the output less readable.

- remove a redundant _updateVariable() in AnimatedContainer (cc
  @abarth).

- add more information to RenderView.toString().

Introduce a class that can describe the metrics used in a Positioned
widget, to enable easier manipulation and storage of the values, e.g.
for animation purposes.

Add type annotations in many places.
Fix some identifiers to have more lint-satisfying names.
Make all operator==s consistent in style.
Reorder some functions for consistency.
Make ParentData no longer dynamic, and fix all the code around that.

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.

Added horizontal and vertical alignment properties to Stack so that the origin of non-positioned children can be specified. Currently all of the non-positioned children just end up with their top-left at 0,0. Now, for example, you can center the children by specifying verticalAlignment: 0.5, horizontalAlignment: 0.5.

Added IndexedStack which only paints the stack child specified by the index property. Since it's a Stack, it's as big as the biggest non-positioned child. This component will be essential for building mobile drop down menus.

Added a (likely temporary) example that demonstrates IndexedStack.

This reverts commit 7592213df29066cd357eaa4fffe4a19ed3bae189.

Turns out many of the functions on BoxConstraints weren't used or had callers
that could easily be updated to other functions. I've added dartdoc to all the
public functions as well as renamed some functions that had similar names but
did different things.

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.

When laying out positioned children inside a stack, we should give them
unbounded constraints because if they draw outside of the stack, we'll just
clip them.

We'll need this for compositing because we need to switch out the sky.Canvas
when we switch compositing layers.

Also, make RenderStack support negative positions with clipping so that we can
expand objects that are partially offscreen.

Previously, we didn't set the width of the child correctly when top and bottom
(or left and right) were both specified.

Fixes #275