Handling of wrong types of arguments in the File API.

runtime/lib/string.dart

Index: runtime/lib/string.dart
diff --git a/runtime/lib/string.dart b/runtime/lib/string.dart
index ca73d05502fe273d55f4c632762cdf1446d61f8a..75f38ec3742912bc8405472298f94790cb5f7a83 100644
--- a/runtime/lib/string.dart
+++ b/runtime/lib/string.dart
@@ -180,7 +180,7 @@ class StringBase {
     return substringUnchecked_(first, last + 1);
   }
 
-  bool contains(Pattern other, int startIndex) {
+  bool contains(Pattern other, [int startIndex = 0]) {
     if (other is String) {
       return indexOf(other, startIndex) >= 0;
     }