Added categories parameter to dat2js handling.

I have added handling for the dart2js parameter 'categories'. This was to handle an issue I was seeing where I was trying to build a polymer code and it was throwing an error message about dart:io. Apparently the fix is to pass 'categories=Server' as a param to dart2j. Added this such that you have the following appears to resolve the problem.

--
				<configuration>
					<checkedMode>true</checkedMode>
					<dartSdk>C:/development/dart/dart-sdk</dartSdk>
					<categories>Server</categories>
				</configuration>
--