join() not working as advertised

[rohintoncollins]

pubspec.yaml:

  path: ^1.9.0

join('/ZCLOUD', 'ZCloud Master Key :grinning:§ąśćżłóę:.txtÆ.txt')

should return

/ZCLOUD/ZCloud Master Key :grinning:§ąśćżłóę:.txtÆ.txt

It is incorrectly returning

ZCloud Master Key :grinning:§ąśćżłóę:.txtÆ.txt

Of note is the emoji 'colon-grinning-colon'
plus the other special characters '§ąśćżłóę'
plus a colon which is a forward slash in the macOS Finder file name (and is represented by a colon)
plus the 'Æ' character.

[lrhn]

Appears to be the behavior of the URL-style, not Posix or Windows.

The URL style most likely considers the second string to be a URI with a scheme, so not a relative path at all, which makes it discard the prior path.
If you check Context(style:Style.url).isAbsolute(...) of the "ZCloud Master...." string, it says true, unlike the other styles.

From the documentation of join:

/// If a part is an absolute path, then anything before that will be ignored:
///
///     p.join('path', '/to', 'foo'); // -> '/to/foo'

Since the second path segment there, using the URL strategy in the context, is absolute, the behavior is correct.

We should consider whether we should improve the isAbsolute for the URL strategy to only recognize actual valid schemes (of the form [a-zA-Z][a-zA-Z0-9+\-.]*:), rather than anything containing a : before a /.

(I'll see if I can make the URI parsing more precise.)