Fix the signature for CryptoUtils.bytesToBase64.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//1346403004 .
diff --git a/lib/src/crypto_utils.dart b/lib/src/crypto_utils.dart
index 8498716..99613ef 100644
--- a/lib/src/crypto_utils.dart
+++ b/lib/src/crypto_utils.dart
@@ -37,7 +37,7 @@
    *
    */
   static String bytesToBase64(List<int> bytes,
-      [bool urlSafe = false, bool addLineSeparator = false]) {
+      {bool urlSafe: false, bool addLineSeparator: false}) {
     return BASE64.encode(bytes,
         urlSafe: urlSafe, addLineSeparator: addLineSeparator);
   }