Change new List(n) to return fixed length list.

sdk/lib/isolate/mangler.dart

Index: sdk/lib/isolate/mangler.dart
diff --git a/sdk/lib/isolate/mangler.dart b/sdk/lib/isolate/mangler.dart
index a0e16721a4d9d0cd220681319342f05cd7555382..8b31d5a621c383d20fee3eb95bc4697372eb7f80 100644
--- a/sdk/lib/isolate/mangler.dart
+++ b/sdk/lib/isolate/mangler.dart
@@ -44,7 +44,7 @@ class _IsolateEncoder {
       for (int i = 0; i < data.length; i++) {
         var mangled = encode(data[i]);
         if (mangled != data[i] && !hasBeenDuplicated) {
-          result = new List.fixedLength(data.length);
+          result = new List(data.length);
           for (int j = 0; j < i; j++) {
             result[j] = data[j];
           }