Context position

Hi. kind Hoa. please take a look.

1. get absolute MouseEvent position from `MouseEvent.client`
2. Move context menu when cropped. the bottom line of context menu will fit `topUi`'s bottom.

Fixes https://github.com/dart-lang/spark/issues/1057
Review @dinhviethoa

TEST = click fileview
![image](https://f.cloud.github.com/assets/756988/2139522/d5385e90-933f-11e3-9c09-f3acf8222e8c.png)
![image](https://f.cloud.github.com/assets/756988/2139537/03153b6c-9340-11e3-8e94-cf7b9a42300b.png)