Add CryptoUtils class.

tests/lib/crypto/sha256_test.dart

Index: tests/lib/crypto/sha256_test.dart
diff --git a/tests/lib/crypto/sha256_test.dart b/tests/lib/crypto/sha256_test.dart
index 588cc575f295c335be40c2b59a48b8e8b76363b9..fe99c4e5fc6647d9a679e55b9401efe95eb2a7b8 100644
--- a/tests/lib/crypto/sha256_test.dart
+++ b/tests/lib/crypto/sha256_test.dart
@@ -19,14 +19,6 @@ List<int> createTestArr(int len) {
   return arr;
 }
 
-String digestToString(List<int> digest) {
-  var buf = new StringBuffer();
-  for (var part in digest) {
-    buf.add("${(part < 16) ? "0" : ""}${part.toRadixString(16).toLowerCase()}");
-  }
-  return buf.toString();
-}
-
 void test() {
   final expected_values = const [
       'e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855',
@@ -288,7 +280,7 @@ void test() {
 
   for (var i = 0; i < expected_values.length; i++) {
     var d = new SHA256().update(createTestArr(i)).digest();
-    Expect.equals(expected_values[i], digestToString(d), '$i');
+    Expect.equals(expected_values[i], CryptoUtils.digestToHex(d), '$i');
   }
 }
 
@@ -307,7 +299,7 @@ void testRepeatedDigest() {
 void testStandardVectors(inputs, mds) {
   for (var i = 0; i < inputs.length; i++) {
     var d = new SHA256().update(inputs[i]).digest();
-    Expect.equals(mds[i], digestToString(d), '$i');
+    Expect.equals(mds[i], CryptoUtils.digestToHex(d), '$i');
   }
 }