- Rename EdgeDims constructor to EdgeDims.TRBL().

- Add operator== to Size and Offset so that you can compare Size to
  DebugSize in checked mode.

- Add Size.lerp().

- Add various operators to EdgeDims. (*, /, ~/, %)

- Add EdgeDims.lerp().

- Update style guide. I went there to fix an EdgeDims constructor
  example, and stayed because some recent things came up and I wanted to
  add them before I forgot.

AnimationPerformance had some logic for supporting multiple variables
that was hardly ever used. ValueAnimation, a subclass, has logic for
handling a single variable. I've removed the logic for handling
variables from AnimationPerformance in favour of most call sites instead
using ValueAnimation.

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.

We removed this value from the engine because it's not useful.

What's important about this code is that it's presenting services outside the
VM, not the particular technology used to present the services.

We now expose idiomatic setters for these properties. Eventually we'll remove
the setter functions.

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

These files really belong on other libraries.