Cautionary tale about optimizing path manipulation.

sdk/lib/_internal/pub/lib/src/io.dart

Index: sdk/lib/_internal/pub/lib/src/io.dart
diff --git a/sdk/lib/_internal/pub/lib/src/io.dart b/sdk/lib/_internal/pub/lib/src/io.dart
index ffbf30e71dcf45da820faa72b097839f38d69773..7f8c9ec088523d54586b963e7da172130c15ab0b 100644
--- a/sdk/lib/_internal/pub/lib/src/io.dart
+++ b/sdk/lib/_internal/pub/lib/src/io.dart
@@ -289,6 +289,16 @@ List<String> listDir(String dir, {bool recursive: false,
     if (entity is Link) return false;
     if (includeHidden) return true;
 
+    // Using substring here is generally problematic in cases where dir has one
+    // or more trailing slashes. If you do listDir("foo"), you'll get back
+    // paths like "foo/bar". If you do listDir("foo/"), you'll get "foo/bar"
+    // (note the trailing slash was dropped. If you do listDir("foo//"), you'll
+    // get "foo//bar".
+    //
+    // This means if you strip off the prefix, the resulting string may have a
+    // leading separator (if the prefix did not have a trailing one) or it may
+    // not. However, since we are only using the results of that to call
+    // contains() on, the leading separator is harmless.
     assert(entity.path.startsWith(dir));
     var pathInDir = entity.path.substring(dir.length);