Remove deprecated string features.

tests/lib/crypto/base64_test.dart

Index: tests/lib/crypto/base64_test.dart
diff --git a/tests/lib/crypto/base64_test.dart b/tests/lib/crypto/base64_test.dart
index fdf24b94b55cd5485e76403914384c220692ae37..0070a50666be48e41f373caf5fc4622308670fad 100644
--- a/tests/lib/crypto/base64_test.dart
+++ b/tests/lib/crypto/base64_test.dart
@@ -45,11 +45,11 @@ var longLineResultNoBreak =
 
 void main() {
   for (var i = 0; i < inputs.length; i++) {
-    var enc = CryptoUtils.bytesToBase64(inputs[i].charCodes);
+    var enc = CryptoUtils.bytesToBase64(inputs[i].codeUnits);
     Expect.equals(results[i], enc);
   }
-  Expect.equals(CryptoUtils.bytesToBase64(longLine.charCodes, 76),
+  Expect.equals(CryptoUtils.bytesToBase64(longLine.codeUnits, 76),
                 longLineResult);
-  Expect.equals(CryptoUtils.bytesToBase64(longLine.charCodes),
+  Expect.equals(CryptoUtils.bytesToBase64(longLine.codeUnits),
                 longLineResultNoBreak);
 }