This moves input.dart to editing2/, since that way we can define the layering as strictly unidirectional.
It also reorders a bunch of imports to fit the style guide.
I removed the old remnants of the widgets example, and put the fn2 examples into the examples/widgets/ directory, to parallel the framework directory names.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1177243002.