Move sky/examples to sky/sdk/lib/example, and code changes to support that change. Fixes T277.

sky/sdk/lib/rendering/README.md

 Sky Rendering
 =============
 
 The Sky render tree is a low-level layout and painting system based on a
 retained tree of objects that inherit from [`RenderObject`](object.dart). Most
 developers using Sky will not need to interact directly with the rendering tree.
 Instead, most developers should use [Sky widgets](../widgets/README.md), which
 are built using the render tree.
 
 Overview
@@ skipping 129 matching lines @@
 
 Writing new subclasses
 ----------------------
 
 ### The RenderObject contract
 
 If you want to define a `RenderObject` that uses a new coordinate
 system, then you should inherit straight from `RenderObject`. Examples
 of doing this can be found in [`RenderBox`](box.dart), which deals in
 rectangles in cartesian space, and in the [sector_layout.dart
-example](../../../examples/rendering/sector_layout.dart), which
+example](../../../sdk/lib/example/rendering/sector_layout.dart), which
 implements a toy model based on polar coordinates. The `RenderView`
 class, which is used internally to adapt from the host system to this
 rendering framework, is another example.
 
 A subclass of `RenderObject` must fulfill the following contract:
 
 * It must fulfill the [AbstractNode contract](../base/README.md) when
   dealing with children. Using `RenderObjectWithChildMixin` or
   `ContainerRenderObjectMixin` can make this easier.
 
@@ skipping 201 matching lines @@
 
 * Avoid using save/restore on canvases.
 
 
 Dependencies
 ------------
 
  * [`package:sky/base`](../base)
  * [`package:sky/mojo`](../mojo)
  * [`package:sky/animation`](../mojo)