Devoncarew refactor testing

Refactor our chrome app networking code (tcp.dart and server.dart) and the chrome app test harness into their own pub packages (currently residing in the top level of this repo). `chrome-app-net.dart` and `chrome-app-testing.dart` respectively.

The testing package uses the networking package in order to test test results back to the test listener server. The networking package uses the testing package in order to run its unit tests in a chrome app.

Spark itself has been refactored to use the networking package. It has not yet been moved over to use the testing package. That will happen in a later CL.

@ussuri, @umop
nit: you mention "test agent" in the comment, but I don't see any class named "TestAgent" or something similar. It would be nice to mention the class/file where this "agent" code is.