Add base 64 encoding to the crypto library utils.

lib/crypto/crypto.dart

Index: lib/crypto/crypto.dart
diff --git a/lib/crypto/crypto.dart b/lib/crypto/crypto.dart
index 7f193c6451aef9b931f059df05084a3e71c945af..a48c252fb13765d68c97951d063cc02271c011c0 100644
--- a/lib/crypto/crypto.dart
+++ b/lib/crypto/crypto.dart
@@ -106,6 +106,14 @@ class CryptoUtils {
   static String bytesToHex(List<int> bytes) {
     return _CryptoUtils.bytesToHex(bytes);
   }
+
+  /**
+   * Converts a list of bytes (for example a message digest) into a
+   * base64 encoded string.
+   */
+  static String bytesToBase64(List<int> bytes) {
+    return _CryptoUtils.bytesToBase64(bytes);
+  }
 }
 
 /**