update file apis

@financeCoding @kevmoo 

This lands some initial work by Kevin to expose the file system objects using the same API as in dart:html. The implementation classes are prefixed with 'Cr'. They're public (not _) in order for them to be instantiated by multiple libraries within chrome_gen. They're not exposed to clients that import chrone_gen however. We selectively export the dart:html types that we rely on (in the files_exp.dart library).

They'll be a follow up CL, to expose some more types, and to run some tests through to make sure things that are exposed are generally working.