Upgrade to Dart 1.6

This upgrades the package to support Dart 1.6. I removed `UriHelper`, since `Uri` has similar functionality for creating `Uri`s from other `Uri`s.

Test Plan:
Run unit tests.

Reviewer: @astashov 
`Uri.toString()` inserts the port number, so I need to mimic how it behaves in the unit test, or the test fails.