Fix: setup.dart failed to run under Linux (#40)

* Fix: setup.dart failed to run under Linux
webcryptoRoot.resolve('src') did not get evaluated to `$PUB_CACHE/webcrypto-x.y.z/src`, but just to `$PUB_CACHE/src`. This broke `flutter pub run webcrypto:setup` at the very least on Linux, but probably on more systems as well. Replacing the URI.resolve approach to a simple string concatenation fixes the bug.

* Avoid potential doubled file seperators

* joinPaths -> _joinPaths

Author: maltemedocs

bin/setup.dart

@@ -63,7 +63,8 @@ void main() async {
     'cmake',
     [
       '-S',
-      webcryptoRoot.resolve('src').toFilePath(),
+      Directory(_joinPaths(webcryptoRoot.toFilePath(), 'src'))
+          .path,
       '-B',
       root.resolve('.dart_tool/webcrypto').toFilePath(),
     ],
@@ -98,3 +99,11 @@ void main() async {
   print('This is only necessary for using package:webcrypto in unit tests');
   print('and scripts, not for usage in applications.');
 }
+
+/// Join paths without duplicate path separators.
+String _joinPaths(String prefix, String suffix) {
+  if (!prefix.endsWith(Platform.pathSeparator)) {
+    prefix += Platform.pathSeparator;
+  }
+  return prefix + suffix;
+}