Merge pull request #807 from Hixie/a11y

Notes on accessibility.

sky/specs/accessibility.md

+Accessibility
+=============
+
+iOS and Android accessibility APIs want to synchronously communicate
+with the main thread, so we need to ship a description of the UI to a
+service on the main thread, from our Dart thread.
+
+On the Dart side, we need a RenderAccessibleBox on the render tree,
+which can be configured with all the various things the accessibility
+APIs might care about (e.g. accessible name, checkedness, etc).
+Separate from the layout and paint phases we have a phase that builds
+an accessibility tree and sends it over to the aforementioned thread.
+When a RenderAccessibleBox's configuration changes, it updates the
+main thread also.
+
+Maybe RenderParagraph also participates in this so that all text is
+automatically exposed.
+
+The main thread can send events like "activate" or "increment slider"
+back to a RenderAccessibleBox, which should expose them somehow.
+
+On the Widget side we'd have an Accessible widget that is a
+OneChildRenderNodeWrapper for RenderAccessibleBox and does what you'd
+expect, maybe also exposing the callbacks.
+
+Components would wrap their interactive parts with these Accessible
+widgets.
+
+Ideally we'd have a way to make the default "activate" action
+automatically turn into a "tap" gesture.