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

pkg/fixnum/lib/src/int64.dart

Index: pkg/fixnum/lib/src/int64.dart
diff --git a/pkg/fixnum/lib/src/int64.dart b/pkg/fixnum/lib/src/int64.dart
index 1addae59a26189ef51a0542fae51e04974a11af8..81607de9b37a26316efefabbe4c1fe2878bd2404 100644
--- a/pkg/fixnum/lib/src/int64.dart
+++ b/pkg/fixnum/lib/src/int64.dart
@@ -625,7 +625,7 @@ class int64 implements intx {
   }
 
   List<int> toBytes() {
-    List<int> result = new List<int>.fixedLength(8);
+    List<int> result = new List<int>(8);
     result[0] = _l & 0xff;
     result[1] = (_l >> 8) & 0xff;
     result[2] = ((_m << 6) & 0xfc) | ((_l >> 16) & 0x3f);