fix test URIs on windows (#8139)
Summary:
Pull Request resolved: https://github.com/facebook/flow/pull/8139
the tests were sending invalid URIs on Windows:
`C:\foo\bar` should become `file:///C%3A/foo/bar`, but was `file:///C:\foo\bar`. when decoded, the backslashes are escaped so this becomes a file named "C:\foo\bar" in the root directory.
since URIs always use forward slashes, `<PLACEHOLDER_PROJECT_URL_SLASH>` is redundant with `<PLACEHOLDER_PROJECT_URL>/`, so I replaced it.
node 10.12 added `url.pathToFileURL` which should do what we want here, but to avoid bumping the required node version, I went with `vscode-uri` instead.
Reviewed By: gabelevi, samwgoldman
Differential Revision:
D18001370
fbshipit-source-id:
15f1cf19c164578ae4747c9c97dc911baeb80bf9