diff --git a/pkg/front_end/test/utils/io_utils.dart b/pkg/front_end/test/utils/io_utils.dart
index a68fef8d455..936d83afdbe 100644
--- a/pkg/front_end/test/utils/io_utils.dart
+++ b/pkg/front_end/test/utils/io_utils.dart
@@ -35,6 +35,20 @@ String computeRepoDir() {
   if (!new Directory(dirPath).existsSync()) {
     throw "The path returned by git ($dirPath) does not actually exist.";
   }
+  if (dirPath.length > 1 && dirPath[1] == ':') {
+    // Absolute Windows path. Normalize drive letter to match Uri.base.
+    if (Uri.base.path.length < 3 ||
+        Uri.base.path[0] != '/' ||
+        Uri.base.path[2] != ':') {
+      throw "Expected Uri.base=${Uri.base} to be an absolute file path.";
+    }
+    bool isLowerCase = Uri.base.path[1] == Uri.base.path[1].toLowerCase();
+    if (isLowerCase) {
+      dirPath = dirPath[0].toLowerCase() + dirPath.substring(1);
+    } else {
+      dirPath = dirPath[0].toUpperCase() + dirPath.substring(1);
+    }
+  }
   return dirPath;
 }