Add Windows Support

[sgerhardt]

It appears the documentation and code are not set to support Windows.

A couple examples:

1. Setting up alias on *nix systems, but no Windows analog
2. Issues wherever Process.run is used
   For example, trying to run tests in analyze_test.dart results in,
   ProcessException: The system cannot find the file specified. Command: pub get

I'm sure there are several more platform issues as well. I'm wondering if there is any desire to make the project more platform independent.

[maxwellpeterson-wf]

None of our developers use Windows, but we would be happy to accept pull requests that add support.

[evanweible-wf]

This should be more achievable with v3, and we can use Travis CI to run our tests on windows.