test large inputs for sha1 and sha256, also remove unused import

test/very_long_input.dart

+library very_long_input_test;
+
+import 'package:crypto/crypto.dart';
+import 'package:test/test.dart';
+
+veryLongInput(digester, int byteLength, String verify) {
+  var nullCharacter = '\u0000'.codeUnitAt(0);
+  var input = [nullCharacter];
+  for (var i = 0; i < byteLength; i++) {
+    digester.add(input);
+  }
+  var d = digester.close();
+  expect(CryptoUtils.bytesToHex(d), verify);
+}