Reland "File::Copy avoids direct copying on Windows"

This is a reland of 391d3bcdd861efde45bd77e9cbdce814cbda3762

The root cause of the failure is that the destination path uses forward
slashes as path separator.

This fix checks for forward slashes as a potential path separators.
If both forward and back slashes exist, the one closer to the end of the
path will be used to get the destination directory.

If any step fails, it will fall back to original copy.


Original change's description:
> File::Copy avoids direct copying on Windows
>
> There is a race condition for copying file on Windows, where CopyFile()
> returns success but data has not been populated into destination file.
> E.g process is killed or died in the middle.
>
> This cl will change File::Copy as
> 1. Copy file to a temp file in the same directory of destination file.
> 2. Rename the file to the target file.
>
> Bug: https://github.com/dart-lang/sdk/issues/42119
> Change-Id: I39b6d451f6ace970bc554501148259d33de232c7
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/149667
> Commit-Queue: Zichang Guo <zichangguo@google.com>
> Reviewed-by: Zach Anderson <zra@google.com>

Bug: https://github.com/dart-lang/sdk/issues/42119
Change-Id: I58c3aa432d3f64bddb1deace4c9a1ceb2f0f5e16
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/151035
Commit-Queue: Zichang Guo <zichangguo@google.com>
Reviewed-by: Zach Anderson <zra@google.com>