Fix invocations of the MD5 constructors.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//1359993003 .
diff --git a/lib/src/hash_sink.dart b/lib/src/hash_sink.dart
index c5986e5..84cdff8 100644
--- a/lib/src/hash_sink.dart
+++ b/lib/src/hash_sink.dart
@@ -9,7 +9,6 @@
 import 'package:typed_data/typed_data.dart';
 
 import 'digest.dart';
-import 'hash.dart';
 import 'utils.dart';
 
 /// A base class for [Sink] implementations for hash algorithms.
diff --git a/lib/src/md5.dart b/lib/src/md5.dart
index 1964bc2..c3d81bc 100644
--- a/lib/src/md5.dart
+++ b/lib/src/md5.dart
@@ -20,7 +20,7 @@
 ///
 /// **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.
 ///
@@ -37,7 +37,7 @@
   @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));