Cleanup warnings/errors from Dart analyzer

[matanlurey]

As of now (9/9/14), there are 52 problems, 295 warnings, and 39 hints from Dart analyzer in a clean external copy of the Streamy master branch. Many are due to using dynamics without casting.

This should be cleaned up to make it easier / less scary for more contributors. I'll contribute, but I could use your help 👍