web components

Hi

I'm hacking around on dart, and was happy to see a plugin for maven. 

I've added some bits to use the web compiler, as this is likely a common usecase. It seems the way dart likes to organise its files is very different to maven, which prefers generated files to live in /target. Not sure how useful it would be to fight dart on that one.

Probably more work needs to be done such as stripping out files not needed to be in the eventual WAR, but a starter for 10...