* justifyContent -> mainAxisAlignment
* alignItems -> crossAxisAlignment
* FlexJustifyContent -> MainAxisAlignment
* FlexAlignItems -> CrossAxisAlignment

Fixes #231

Fixes #1382

Now we just have one TextSpan class that handles both simple strings, trees of
children, and styling both. This approach simplifies the interface for most
clients.

This patch also removes StyledText, which was weakly typed and tricky to use
correctly. The replacement is RichText, which is strongly typed and uses
TextSpan.

Our examples have been growing organically over time. This patch cleans
them up to illustrate specific aspects of Flutter.

This makes skyanalyzer also check the examples, and fixes everything it
found there.

These are now declared in dart:sky as part of ParagraphBuilder.

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.

Sadly, box.dart has grown much longer than 1000 lines. This patch splits it up
into several files based on the class hierarchy. Fortunately, many of these
classes are loosely coupled to each other.

We had a remarkable number of analyzer failures.

I'll fix the bots to analyze across the whole project
in a follow-up patch, that should prevent this
in the future.

@abarth

 - Inline -> Pargraph. This class is actually a box, not an inline. It's really
   a wrapper for RenderParagraph, so Paragraph is the normal name.

 - InlineBase -> RenderInline. The name we used in C++ for the base class of
   all inlines was RenderInline, which removes the ugly "Base" suffix.

 - InlineText -> RenderText. Aligns this name with C++.

 - InlineStyle -> RenderStyled. Matches the foregoing pattern.