- add a checkbox to the stock app, so that we're testing the checkbox widget
  (it's not currently wired up to anything, that can come later)
- make InkSplash use FlexContainer so that we can use flex in the popup menu items
- make effen's Text be more similar to Image and Container, so that it can be styled
- make layout.dart's RenderCSSText correctly support being styled
- also fixes a bug with the stock list where we were rendering one too few a row when scrolling
- check in the code to dump the DOM so I don't have to keep remembering how to do this

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1134163003