Remove faulty function return type in builtin.dart

runtime/bin/builtin.dart

Index: runtime/bin/builtin.dart
diff --git a/runtime/bin/builtin.dart b/runtime/bin/builtin.dart
index 70abc70fbe3b613c72cff6db54fbcf7ccba32fc0..be17c3f919576b178b06d3748da33e5a68efeb43 100644
--- a/runtime/bin/builtin.dart
+++ b/runtime/bin/builtin.dart
@@ -266,7 +266,7 @@ String _filePathFromPackageUri(Uri uri) {
 // The directory part is either a file system path or an HTTP(S) URL.
 // The filename part is the extension name, with the platform-dependent
 // prefixes and extensions added.
-String _extensionPathFromUri(String userUri) {
+_extensionPathFromUri(String userUri) {
   if (!userUri.startsWith(_DART_EXT)) {
     throw 'Unexpected internal error: Extension URI $userUri missing dart-ext:';
   }