Updated the streams tutorials to use UTF8.decoder instead of StringDecoder

Updated the streams tutorials to use UTF8.decoder instead of the deprecated StringDecoder. 

Reference link about StringDecoder change: 
http://stackoverflow.com/questions/18927667/what-happened-to-stringdecoder-in-dart

Note: I have signed the CLA but I think it's waiting to be processed (https://developers.google.com/open-source/cla/individual?csw=1)