Add base 64 encoding to the crypto library utils.

lib/crypto/crypto_vm.dart

Index: lib/crypto/crypto_vm.dart
diff --git a/lib/crypto/crypto_vm.dart b/lib/crypto/crypto_vm.dart
index d7f36403a1fb9fd18d0dde88fc937dc333d0811d..89f865835f3237d4bc4b7138553371697af36850 100644
--- a/lib/crypto/crypto_vm.dart
+++ b/lib/crypto/crypto_vm.dart
@@ -102,6 +102,14 @@ 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);
+  }
 }
 
 /**