Updated the streams and cmdline tutorials

1. 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

2. Updated cmdline tutorial to reflect the breaking changes done in commit r31468 where Platform.environment was changed to return an unmodifiable map. Reference link to Dart Misc: https://groups.google.com/a/dartlang.org/d/topic/misc/fYe5BgfrVHc/discussion .

Note: I have signed the CLA but I think it's waiting to be processed (I used the form here https://developers.google.com/open-source/cla/individual?csw=1)