File operations aren't Windows-friendly

[nex3]

It looks like grinder_files.dart assumes that each platform has a single path separator that's used uniformly in all paths:

grinder.dart/lib/grinder_files.dart

Lines 116 to 119 in b335e3a

	String get name {
	int index = _path.lastIndexOf(_sep);
	return index != -1 ? _path.substring(index + 1) : null;
	}

This is not true on Windows, where \ and / may both be used as separators, including within the same path. This leads to bugs where, for example, the copy() function can incorrectly include the source file's directory name in the destination path.

I recommend using the path package rather than rolling custom path manipulation.