Test (and fix) behavior of MemoryFile on Windows when the file is moved while open

[jamesderlin]

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In https://github.com/google/file.dart/pull/157, I made MemoryFile/MemoryRandomAccessFile try to handle the file being renamed/moved/removed from under them. Currently they always follow POSIX behavior (the equivalent of the open file handle tracks the equivalent of the inode), which is consistent with how dart:io File behaves on POSIX systems.

I have not verified what happens on Windows systems, but I expect that rename/move/remove operations on dart:io's File will fail with an exception if they're attempted on files with open handles. If so, MemoryFile should follow the behavior and would need to keep track of openRead/openWrite/open/openAsync calls (and determine when the corresponding Stream/IOSink/MemoryRandomAccessFile is closed) and disallow the file from being renamed/moved/removed. (For completeness I think we'd also need to prevent such operations on all parent directories too.)

Since one might want to test behavior for a platform other what a test is running on, @dnfield suggested that we could make the behavior configurable based on the FileSystemStyle.