Fix invocations of the MD5 constructors.

lib/src/md5.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library crypto.md5;
 
 import 'dart:convert';
 import 'dart:typed_data';
 
 import 'digest.dart';
 import 'hash.dart';
 import 'hash_sink.dart';
 import 'utils.dart';
 
 /// An instance of [MD5].
 ///
 /// This instance provides convenient access to the [MD5][rfc] hash function.
 ///
 /// [rfc]: https://tools.ietf.org/html/rfc1321
 ///
 /// **Warning**: MD5 has known collisions and should only be used when required
 /// for backwards compatibility.
-final md5 = new MD5._();
+final md5 = new MD5();
 
 /// An implementation of the [MD5][rfc] hash function.
 ///
 /// [rfc]: https://tools.ietf.org/html/rfc1321
 ///
 /// **Warning**: MD5 has known collisions and should only be used when required
 /// for backwards compatibility.
 ///
 /// Note that it's almost always easier to use [md5] rather than creating a new
 /// instance.
 class MD5 extends Hash {
   final int blockSize = 16 * bytesPerWord;
 
   @Deprecated("Use the md5 field instead.")
   MD5();
 
-  MD5 newInstance() => new MD5._();
+  MD5 newInstance() => new MD5();
 
   ByteConversionSink startChunkedConversion(Sink<Digest> sink) =>
       new ByteConversionSink.from(new _MD5Sink(sink));
 }
 
 /// Data from a non-linear mathematical function that functions as
 /// reproducible noise.
 const _noise = const [
   0xd76aa478, 0xe8c7b756, 0x242070db, 0xc1bdceee, 0xf57c0faf, 0x4787c62a,
   0xa8304613, 0xfd469501, 0x698098d8, 0x8b44f7af, 0xffff5bb1, 0x895cd7be,
@@ skipping 69 matching lines @@
               _shiftAmounts[i]));
       a = temp;
     }
 
     digest[0] = add32(a, digest[0]);
     digest[1] = add32(b, digest[1]);
     digest[2] = add32(c, digest[2]);
     digest[3] = add32(d, digest[3]);
   }
 }