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

Fixes #1638

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.

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.

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.

Also, add asserts that the text has layout before being painted.

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.

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

Previously, no text was appearing because we weren't actually converting the
Dart objects into DOM.

 - 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.

We were applying the style to the RenderInline but we actually needed to apply
it to the RenderParagraph. The lineHeight property had the same problem.