Add the ability to test a FileSystem without using dart:io explicitly

[brianquinlan]

The idea is that a non-platform FileSystem implementation (e.g. in-memory, WebDAV, etc.) should be testable using only itself as a reference.

I only updated only test to show what the change would look like conceptually.

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[aam]

newline separator?

[brianquinlan]

I'll add it when I need it (and a bunch of other stuff ;-))