Remove errors and some warnings to look nice in Dart Editor.

With r40722 Dart Editor will support async-await syntax (enabled from config file, not UI).

I made a few edits to this package to remove errors and (some) warnings. The folders named simplified* have to be marked 'Don't Analyze' in the editor.

I'm hoping to use this package in some new large-scale tests for chromedeveditor.

I had to add a couple dev dependencies that are path-based. Those paths probably won't work as-is for others, but I'm not sure of a better way to define them.